Slow forking is only the second biggest problem IMO. The biggest is the lack of proper signals. There's a bunch of software out there that just isn't architected to work well without non-cooperative preemption.
That's fake cooperative emulation of signals. It isn't preemptive (unless someone got a kernel driver approved while I wasn't looking?) thus many things either work poorly or not at all. Pause-the-world GC algorithms are a good example. Coroutine implementations also have to be cooperative.
If you're curious, I believe the issue was discussed at length in the Go GitHub issues years ago. Also on the mailing lists of many other languages.