• The only way to limit handle inheritance in performant way on apple targets is to use POSIX_SPAWN_CLOEXEC_DEFAULT supported by posix_spawn. So it's a must have for Make Process.Start have a option to change handle inheritance #13943.
• It would allow us to support Process APIs on iOS. The only unclear part is testing.
• It's the recommended and most performant way to spawn processes on Apple targets. The numbers I got for Prefer posix_spawn on OSX #126063 show even up to x100 boost on arm64 and x5 on x64.
• We can't take for granted that it would just work on all the platforms. For example in Don't use vfork on android #118085, we had to switch to fork (from vfork) on Android. However, it's worth just asking Copilot to write a prototype and use EgorBot to run some simple benchmarks on Linux x64 and arm64. We should benchmark various scenarios that would include processes that are cheap and expensive to copy . Check https://rust-lang.github.io/rfcs//3228-process-process_group.html for rust approached it.
• We still need a fallback to fork/exec to support setguid, setuid and groups:

cc @jkotas @stephentoub @tmds