Interesing idea. I am wondering what are the use cases on top of your head? I am asking because in my understanding people who care concurrency and parallelism are often those who care performance.
Parallel/threads is some whole other can of worms, of course. It is unfortunate that the stdlib is weak, both here and for numerics, and for other things, and that people are as dependency allergic as in C culture.
Anyway, "easier to optimize" is often subjective, and I don't mean to discourage you.
Multithreading in Nim is bad to say the least and has been for a while. The standard library option for multithreading is deprecated, and most alternatives like weave are either unmaintained or insanely limited (taskpools, malebolgia, etc.). There's no straightforward, idiomatic way to write data-parallel or task-parallel code in Nim today.
The idea of the project is to make shared-memory parallelism as simple as writing a `parallel:` block, without macros or unsafe hacks, and with synchronization handled automatically by the compiler.
Course, performance can be dragged out of Nim with effort, but there's a need for a language where fast, concurrent, GC-optional code is the default, not something one has to wrestle into existence.
I'm curious if you have looked at Chapel: https://chapel-lang.org/
If not, you might find some thing there inspiring.