diff --git a/.eslintrc.json b/.eslintrc.json
index de10df0..354f900 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -34,7 +34,8 @@
         "@typescript-eslint/consistent-type-imports": "off",
         "@typescript-eslint/ban-types": "off",
         "sonarjs/prefer-single-boolean-return": "off",
-        "unicorn/no-typeof-undefined": "off" // todo disable globally
+        "unicorn/no-typeof-undefined": "off", // todo disable globally
+        "@typescript-eslint/consistent-type-definitions": "off"
     },
     "overrides": [
         {
diff --git a/typescript/src/completions/boostNameSuggestions.ts b/typescript/src/completions/boostNameSuggestions.ts
index 2ce3a56..7a919ef 100644
--- a/typescript/src/completions/boostNameSuggestions.ts
+++ b/typescript/src/completions/boostNameSuggestions.ts
@@ -1,3 +1,4 @@
+import { cachedResponse } from '../decorateProxy'
 import { boostExistingSuggestions, boostOrAddSuggestions, findChildContainingPosition } from '../utils'
 import { getCannotFindCodes } from '../utils/cannotFindCodes'
 
@@ -46,7 +47,7 @@ export default (
     }
 
     if (filterBlock === undefined) return entries
-    const semanticDiagnostics = languageService.getSemanticDiagnostics(sourceFile.fileName)
+    const semanticDiagnostics = cachedResponse.getSemanticDiagnostics?.[sourceFile.fileName] ?? []
 
     const notFoundIdentifiers = semanticDiagnostics
         .filter(({ code }) => cannotFindCodes.includes(code))
diff --git a/typescript/src/decorateProxy.ts b/typescript/src/decorateProxy.ts
index 5e9a034..64af965 100644
--- a/typescript/src/decorateProxy.ts
+++ b/typescript/src/decorateProxy.ts
@@ -33,6 +33,10 @@ export const getInitialProxy = (languageService: ts.LanguageService, proxy = Obj
     return proxy
 }
 
+export const cachedResponse = {
+    getSemanticDiagnostics: {} as Record<string, ts.Diagnostic[]>,
+}
+
 export const decorateLanguageService = (
     { languageService, languageServiceHost }: PluginCreateArg,
     existingProxy: ts.LanguageService | undefined,
@@ -169,11 +173,19 @@ export const decorateLanguageService = (
                           .filter(([, v]) => v === false)
                           .map(([k]) => k)
             if (toDisable.includes(feature)) return undefined
+
             // eslint-disable-next-line @typescript-eslint/no-require-imports
             const performance = globalThis.performance ?? require('perf_hooks').performance
             const start = performance.now()
+
             //@ts-expect-error
             const result = orig(...args)
+
+            if (feature in cachedResponse) {
+                // todo use weakmap with sourcefiles to ensure it doesn't grow up
+                cachedResponse[feature][args[0]] = result
+            }
+
             const time = performance.now() - start
             if (time > 100) console.log(`[typescript-vscode-plugin perf warning] ${feature} took ${time}ms: ${args[0]} ${args[1]}`)
             return result
diff --git a/typescript/test/completions.spec.ts b/typescript/test/completions.spec.ts
index 6abcd2b..9e95a59 100644
--- a/typescript/test/completions.spec.ts
+++ b/typescript/test/completions.spec.ts
@@ -40,6 +40,19 @@ test('Banned positions', () => {
     expect(getCompletionsAtPosition(cursorPositions[2]!)?.entries).toHaveLength(1)
 })
 
+test.todo('Const name suggestions (boostNameSuggestions)', () => {
+    const tester = fourslashLikeTester(/* ts */ `
+    const /*0*/ = 5
+    testVariable
+    `)
+    languageService.getSemanticDiagnostics(entrypoint)
+    tester.completion(0, {
+        includes: {
+            names: ['testVariable'],
+        },
+    })
+})
+
 test('Banned positions for all method snippets', () => {
     const cursorPositions = newFileContents(/* tsx */ `
         import {/*|*/} from 'test'
diff --git a/typescript/test/testing.ts b/typescript/test/testing.ts
index 8faad6c..d7ecce8 100644
--- a/typescript/test/testing.ts
+++ b/typescript/test/testing.ts
@@ -27,7 +27,7 @@ interface CodeActionMatcher {
 
 const { languageService, languageServiceHost, updateProject, getCurrentFile } = sharedLanguageService
 
-const fakeProxy = {} as Pick<typeof languageService, 'getApplicableRefactors' | 'getEditsForRefactor'>
+export const fakeProxy = {} as Pick<typeof languageService, 'getApplicableRefactors' | 'getEditsForRefactor'>
 
 codeActionsDecorateProxy(fakeProxy as typeof languageService, languageService, languageServiceHost, defaultConfigFunc)
 
@@ -82,7 +82,7 @@ export const fourslashLikeTester = (contents: string, fileName = entrypoint, { d
 
     const ranges = positive.reduce<number[][]>(
         (prevRanges, pos) => {
-            const lastPrev = prevRanges[prevRanges.length - 1]!
+            const lastPrev = prevRanges.at(-1)!
             if (lastPrev.length < 2) {
                 lastPrev.push(pos)
                 return prevRanges
@@ -92,58 +92,68 @@ export const fourslashLikeTester = (contents: string, fileName = entrypoint, { d
         [[]],
     )
     return {
-        completion: (marker: number | number[], matcher: CompletionMatcher, meta?) => {
-            for (const mark of Array.isArray(marker) ? marker : [marker]) {
-                if (numberedPositions[mark] === undefined) throw new Error(`No marker ${mark} found`)
-                const result = getCompletionsAtPosition(numberedPositions[mark]!, { shouldHave: true })!
-                const message = ` at marker ${mark}`
-                const { exact, includes, excludes } = matcher
-                if (exact) {
-                    const { names, all, insertTexts } = exact
-                    if (names) {
-                        expect(result?.entryNames, message).toEqual(names)
-                    }
-                    if (insertTexts) {
-                        expect(
-                            result.entries.map(entry => entry.insertText),
-                            message,
-                        ).toEqual(insertTexts)
-                    }
-                    if (all) {
-                        for (const entry of result.entries) {
-                            expect(entry, entry.name + message).toContain(all)
-                        }
-                    }
-                }
-                if (includes) {
-                    const { names, all, insertTexts } = includes
-                    if (names) {
-                        for (const name of names) {
-                            expect(result?.entryNames, message).toContain(name)
+        completion(marker: number | number[], matcher: CompletionMatcher, meta?) {
+            const oldGetSemanticDiagnostics = languageService.getSemanticDiagnostics
+            languageService.getSemanticDiagnostics = () => {
+                throw new Error('getSemanticDiagnostics should not be called because of performance reasons')
+                // return []
+            }
+
+            try {
+                for (const mark of Array.isArray(marker) ? marker : [marker]) {
+                    if (numberedPositions[mark] === undefined) throw new Error(`No marker ${mark} found`)
+                    const result = getCompletionsAtPosition(numberedPositions[mark]!, { shouldHave: true })!
+                    const message = ` at marker ${mark}`
+                    const { exact, includes, excludes } = matcher
+                    if (exact) {
+                        const { names, all, insertTexts } = exact
+                        if (names) {
+                            expect(result?.entryNames, message).toEqual(names)
                         }
-                    }
-                    if (insertTexts) {
-                        for (const insertText of insertTexts) {
+                        if (insertTexts) {
                             expect(
                                 result.entries.map(entry => entry.insertText),
                                 message,
-                            ).toContain(insertText)
+                            ).toEqual(insertTexts)
+                        }
+                        if (all) {
+                            for (const entry of result.entries) {
+                                expect(entry, entry.name + message).toContain(all)
+                            }
                         }
                     }
-                    if (all) {
-                        for (const entry of result.entries.filter(e => names?.includes(e.name))) {
-                            expect(entry, entry.name + message).toContain(all)
+                    if (includes) {
+                        const { names, all, insertTexts } = includes
+                        if (names) {
+                            for (const name of names) {
+                                expect(result?.entryNames, message).toContain(name)
+                            }
+                        }
+                        if (insertTexts) {
+                            for (const insertText of insertTexts) {
+                                expect(
+                                    result.entries.map(entry => entry.insertText),
+                                    message,
+                                ).toContain(insertText)
+                            }
+                        }
+                        if (all) {
+                            for (const entry of result.entries.filter(e => names?.includes(e.name))) {
+                                expect(entry, entry.name + message).toContain(all)
+                            }
                         }
                     }
-                }
-                if (excludes) {
-                    for (const exclude of excludes) {
-                        expect(result?.entryNames, message).not.toContain(exclude)
+                    if (excludes) {
+                        for (const exclude of excludes) {
+                            expect(result?.entryNames, message).not.toContain(exclude)
+                        }
                     }
                 }
+            } finally {
+                languageService.getSemanticDiagnostics = oldGetSemanticDiagnostics
             }
         },
-        codeAction: (marker: number | number[], matcher: CodeActionMatcher, meta?, { compareContent = false } = {}) => {
+        codeAction(marker: number | number[], matcher: CodeActionMatcher, meta?, { compareContent = false } = {}) {
             for (const mark of Array.isArray(marker) ? marker : [marker]) {
                 if (!ranges[mark]) throw new Error(`No range with index ${mark} found, highest index is ${ranges.length - 1}`)
                 const start = ranges[mark]![0]!
@@ -192,10 +202,10 @@ export const fileContentsSpecialPositions = (contents: string, fileName = entryp
         let mainMatch = currentMatch[1]!
         if (addOnly) mainMatch = mainMatch.slice(0, -1)
         const possiblyNum = +mainMatch
-        if (!Number.isNaN(possiblyNum)) {
-            addArr[2][possiblyNum] = offset
-        } else {
+        if (Number.isNaN(possiblyNum)) {
             addArr[mainMatch === 't' ? '0' : '1'].push(offset)
+        } else {
+            addArr[2][possiblyNum] = offset
         }
         replacement.lastIndex -= matchLength
     }