Try speeding up CI

[IAlibay]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.27%. Comparing base (ecc8a50) to head (fec5ee1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
- Coverage   93.98%   91.27%   -2.71%     
==========================================
  Files         134      134              
  Lines        9790     9790              
==========================================
- Hits         9201     8936     -265     
- Misses        589      854     +265     

Flag	Coverage Δ
fast-tests	91.27% <ø> (?)
slow-tests	?

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[IAlibay]

Experiment 1: enforcing cpu threads to 1 doesn't seem to have any effect, could even be detrimental.
Experiment 2: removing coverage on long tests shaves off ~ 1/3 of the time.

[IAlibay]

Ok current state gets us to ~ 50% less, which at least gets rid of more immediate worries about 5h long runners. Merge will still be a problem though :/