1. What happened
When using IL2CPP backend the ThreadPool.QueueUserWorkItem() multithreading method leads to slower performances for each thread created while Mono backend works as expected.

2. How we can reproduce it using the example you attached
I wrote a little script using multithread to dispatch a stupid algorithm incrementing an array of int. You can see threadcount, threadsize, average total duration in game view as well as estimated average time for each thread execution.

By building with Mono backend you'll notice that pressing "X" on the pad will change threadcount, globaly the more thread you set the faster the array is completed.

Then switch to IL2CPP and notice that even if performances are great using this backend, the multithreading support seems to destroy every gain and the more thread you use the slower the method is.