Feature Request: Cell Execution Time Display

[archpulse]

Feature Request: Cell Execution Time Display

📝 Description

Add a non-intrusive execution time indicator that displays the duration of each cell execution directly in the notebook UI. This helps users identify performance bottlenecks and optimize slow cells at a glance.

🎯 Why This Is Valuable

1. Performance visibility — Instantly see which cells are slow without profiling tools
2. Learning & debugging — Identify bottlenecks when optimizing code
3. User feedback — Reassurance that long-running cells are actually executing
4. Data quality — Spot unexpected slowdowns that might indicate issues
5. Minimal overhead — Uses Python's time.perf_counter() (virtually no cost)

💡 Implementation Details

Backend

• Wrap cell execution in a timer (using time.perf_counter())
• Send execution time as metadata with cell output
• Format: { cellId, executionTime_ms }

Frontend

• Display small badge in cell header: ⏱️ 245ms
• Color coding:
  • Green (< 100ms): fast ✅
  • Yellow (100ms - 1s): normal ⚠️
  • Orange (1s - 5s): slow 🟠
  • Red (> 5s): very slow 🔴
• Optional: tooltip showing detailed timing on hover

Example UI

✨ Benefits

• No breaking changes — Purely additive feature
• Lightweight — Minimal performance impact
• Visual appeal — Makes marimo feel more "responsive"
• Batteries-included — Fits marimo's philosophy of including useful tools out-of-the-box
• Community request — Common in other notebook tools (Jupyter, VS Code, Pluto.jl)

🔧 Estimated Scope

• Backend: ~50 lines (timer logic)
• Frontend: ~80 lines (React component + styling)
• Tests: ~40 lines
• Total: ~150 lines (very small change)

📌 Related

This complements marimo's existing reactive system by adding transparency to execution performance.

[stephenlf]

I like this, but without the color grading. Too busy.