I cannot see any code so it is hard to imagine what could be going on exactly.

but in general you should be able to monkey patch about anything in javascript as far as I know.

exactly where is the entry point for mocha I do not know, but for example you should be able to assign your custom implementation to any object on any fuction

like even this should be legal, for how it should work I guess you could go read source. I have no idea precisely.

performance.now=()=>{
  //do stuff
}