Long story short, heap allocation is painfully slow. Any sort of malloc will always be slower than a custom pool or a bump allocator, because it has a lot more context to deal with.
Rust makes it especially hard to use custom allocators, see bumpalo for example [0]. To be fair, progress is being made in this area [1].
Theoretically one can use a "handle table" as a replacement for pools, you can find relevant discussion at [2].