The trick with scalable threading is to make the behaviour as independent as is possible. Including memory allocations. “jemalloc” been something worth looking at.

It’s better to follow the scatter/gather model of MPI (don’t use that) in the same process and do as much amortisation in each thread as possible before collation.

When it comes to I/O (with spinning rust), a measured rule is to use twice the cores to account for seek time and use big writes/reads rather than small ones. This can saturate the disks so do measure it.

I spent about ten years worrying about these kind of issues on enterprise storage in C++.