In the wiki, it says this:

What do you mean by

and may have an impact on in-game performance.

?

Does it limit the threads the game can use while it's running? That would be quite undesirable and limit performance, wouldn't it?

The main worker executor is used by resource loading, and setting a lower thread count will make loading slower but it does not make the game run slower. What I mean by it affecting in-game performance is that some loading tasks might be ran while the in game, or some mods might use this executor to run stuff. So setting it to a lower number might create more stutters while playing, but it generally shouldn't affect performance that much.