On linux fork is pretty fast since it's copy on write. Exec or anything similar is gonna be slower. For example you want to exec some program in the elf format. Well exec is likely to lead these things: One is likely got to load the file from non-volatile storage. Then parse the elf file and place each section in the correct place in memory, setup other memory maps, and stack ect.. Not just that though then run time linker probably has to run loading and linking each .so file, also patching code with the correct relocations as needed. Loading a new process is a lot more expensive where as fork can reuse a lot of that work since it's just a clone of the process at the point fork was called and was already done.

vfork() / CLONE_VFORK / posix_spawn()