There isn’t an “ECS library.”
ECS is a paradigm - a way to structure data and logic.
What you’re calling “ECS libraries” are just wrappers stacked on wrappers, and every wrapper is overhead.
Luau doesn’t have macros, a linker, or compile-time specialization like C/C++, so a true zero-cost ECS cannot exist as a library.
The moment you abstract it, you’re already losing the whole point.
If you want real ECS, you write it directly.
If you want wrappers… that’s just OOP with a funny hat.