performance: revert ioThreads value to 3.x

[jknack]

The performance test https://www.techempower.com/benchmarks/#section=test&runid=809d8655-c602-42a1-9d8c-dc4692738790&l=zhuqrj-cn3&test=plaintext for plainText didn't went so good. There is a degradation compared to 3.x which makes some internals optimization useless.

Going to revert the ioThreads default to previous version to see if that helps.

[kliushnichenko]

Just for clarity.
Benchmark is on 4.0.2

IO/Worker threads defaults fix was merged into 4.0.4

[jknack]

You're right. I forgot about your PR, still I found the regression

[jknack]

@kliushnichenko which OS do you run? On OSX found the ioThreads=cpu to run faster then cpu*2

[jknack]

nevermind