i thought this was all fixed with special modes of clone that are optimized and don't actually copy anything (ie, it creates a new deficient process that can pretty much only exec)?
Kind of. Those exist, but because Linux’s formal ABI is syscalls and not libraries that combine them in known-safe ways, the clone speedups that make fork faster are a confusing and fragile API for low-level programmers to use.
That, and even those clone-without-pagetable-copy improvements leave a lot of slowness on the table. Being able to skip even disable-able functionality intended for fork would simplify code. Also, for programs that launch the same subprocess many times, a better API might allow caching away some of the pre-entrypoint initialization of exec.
