docs: add benchmark result for encoder on cpu

[bxue-l2]

Why are these changes needed?

As title

Checks

• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.40%. Comparing base (61019b4) to head (ec2ce8a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2519   +/-   ##
=======================================
  Coverage   39.40%   39.40%           
=======================================
  Files         537      537           
  Lines       48502    48502           
=======================================
+ Hits        19110    19111    +1     
+ Misses      26999    26998    -1     
  Partials     2393     2393           

Flag	Coverage Δ
litt-tests	32.99% <ø> (ø)
unit-tests	40.29% <ø> (+<0.01%)	⬆️

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