I don't think you can turn off all mitigations, since the retpoline (https://stackoverflow.com/questions/48089426/what-is-a-retpo...) stuff is generated by the compiler. You'd probably need to recompile the kernel. How much retpoline is responsible for the performance hit, I don't know.

Also the kernel flags, don't block intel microcode "improvements". You have to do this with:

  sudo apt-mark hold intel-microcode

and look for what's currently installed.

Mitigation isn't a fix, it's a bandaid.