What can be slow are context switches, but they aren't slow in absolute terms. The vast majority of applications, including Web servers, are perfectly fine with 1:1 threading.
Also, if nothing else, you'll run out of PID numbers, as they're usually still 16 bits, even today, though there was a kernel compilation option to change that, from what I remember.
/proc/sys/kernel/pid_max
> PID_MAX_LIMIT, approximately 4 million
/proc/sys/kernel/threads-max
> FUTEX_TID_MASK (0x3fffffff), [approximately 1 billion]
For per-process limit, increase RLIMIT_NPROC.
4K for kernel stacks, only 2K with future work. That's really not much space at all if you're doing anything interesting with those threads.
> Also, if nothing else, you'll run out of PID numbers, as they're usually still 16 bits, even today
Not for a very long time.