[async] Improve benchmarks

[yuanming-hu]

Related issue = #742

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[codecov]

Codecov Report

Merging #2005 into master will increase coverage by 0.94%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2005      +/-   ##
==========================================
+ Coverage   42.57%   43.51%   +0.94%     
==========================================
  Files          45       45              
  Lines        6474     6264     -210     
  Branches     1109     1109              
==========================================
- Hits         2756     2726      -30     
+ Misses       3544     3365     -179     
+ Partials      174      173       -1     

Impacted Files	Coverage Δ
python/taichi/lang/__init__.py	40.57% <0.00%> (-0.33%)	⬇️
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/testing.py	75.00% <0.00%> (-0.72%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/misc/util.py	20.54% <0.00%> (-0.21%)	⬇️
python/taichi/misc/task.py	0.00% <0.00%> (ø)
python/taichi/tools/patterns.py	0.00% <0.00%> (ø)
python/taichi/lang/kernel.py	73.00% <0.00%> (+0.16%)	⬆️
python/taichi/misc/gui.py	8.89% <0.00%> (+0.18%)	⬆️
... and 9 more

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[k-ye]

Great!!