perf: Increase default async thread count for low core count systems

[nameexhaustion]

Removes the division by 4 before clamping when calculating the async thread pool size.

This may benefit low core count systems, specifically those with less than 16 cores. Any potential performance gains may be observed particularly in systems with up to 7 cores, as the difference between having 1 vs 2 async threads is the most substantial.

rayon threadpool size	old async thread count	new async thread count
1	1	1
2	1	2
3	1	3
4	1	4
5	1	4
6	1	4
7	1	4
8	2	4
9	2	4
10	2	4
11	2	4
12	3	4
13	3	4
14	3	4
15	3	4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.34%. Comparing base (245d549) to head (841e5ff).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19829   +/-   ##
=======================================
  Coverage   79.33%   79.34%           
=======================================
  Files        1548     1548           
  Lines      214193   214193           
  Branches     2460     2460           
=======================================
+ Hits       169937   169945    +8     
+ Misses      43698    43690    -8     
  Partials      558      558           

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