I don’t think writing to and reading from disk is a significant portion of this loop. You’d want to measure the cost and thus maximum benefit. SSDs are fast. There isn’t much upside here.
The problem may be disk speed, but I agree it may not. The problem I see is the dropping of state.
Specifically, to produce a .o file the compiler has already read through and created indexes of module::function/struct names, their layouts, dependencies, etc.
My understanding is that the .o files need to be re-parsed by the linker to create these indexes, layouts, and dependencies. Especially with LTOs I'd imagine there would be additional inlining work (stuff the compiler is already good at).
This is all just wasted time, including the IO bottlenecks - even if those are marginal.
