Optimize check_checksum_field

[KristofferC]

There is no point checking this condition in every loop body when we can just split up the loop into a couple with "constant" bodies. This was decently hot in my profiling. This makes it about 7x faster and have around a 10% end to end difference in my use case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (9dd8ed1) to head (0e74316).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
+ Coverage   97.41%   97.55%   +0.13%     
==========================================
  Files           4        4              
  Lines         812      817       +5     
==========================================
+ Hits          791      797       +6     
+ Misses         21       20       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.