Damn and I still remember when Pentium 4 is the epitome of speed and I have to make do with Pentium 3 and even 2.
Pentium 4 was the epitome of heat. The Athlon XP was generally faster and cheaper during that era.
3 years ago I tried to run any modern distro on Pentium 3 (without compiling anything by myself). It appeared I wasn't able due to "invalid opcode" error inside systemd. I switched to Devuan (Debian fork without systemd) and it boot and was as usable as first raspberry pie.
I think you'd want to aim somewhere around the Pentium 4 / Athlon XP era. The docs say it doesn't support the original Pentium, so I suppose you could go back as far as the Pentium II if you really want to suffer.