perf: Reduce default async thread count

[nameexhaustion]

Work spawned onto tokio threads are generally lightweight I/O tasks that spend most of their time awaiting, so we don't need a lot of threads for it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.33%. Comparing base (c4c5224) to head (25c53cd).
Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18142      +/-   ##
==========================================
- Coverage   80.35%   80.33%   -0.03%     
==========================================
  Files        1496     1496              
  Lines      197754   197761       +7     
  Branches     2822     2822              
==========================================
- Hits       158911   158875      -36     
- Misses      38321    38364      +43     
  Partials      522      522              

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