My recollections are that you needed about P-133 to guarantee smooth MPEG1 playback with no hardware support. With full decoders it could be done on fast 386, with halfass acceleration, scaling support etc, you could just do it on a hotrod 486 100Mhz+.
For MPEG2, there was a bit of a hump in software support in that the software was about 25% slower in first couple of years then got optimisations and sped up a bit, so you could probably get a PII 300 doing it okay now where it wouldn't quite do it when new. Same kind of thing for MPEG1 software too, might have jumped ahead with pentium optimisations. Anyway, K6 class can manage it somewhere in the 300-400 range usually, but depends on bus speed and setup. Hardware support varies, I think a full separate decoder would have you doing good on a P-133 again, whereas inbuilt to graphics card, Rage II, Sis6326 etc decoders might have had you needing 200Mhz to pull it off.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.