Were there actually Pentium M chipsets that could decode anything but MPEG2?

The CPU will be struggling with most modern video formats including h.264.

we were decoding 480x320 MP4 on PalmOS 5 devices in early 2000. Those were single-core in-order 200mhz ARM devices with no accelerators at all. Pentium M outperforms those easily and thus can do it too.

Mp4 is the container. H264 is the video codec.

got me, it was DivX and XviD which are indeed newer and fancier than MPEG2

And still much easier to play than h264. A Pentium II with NetBSD was more than enough.

Nowadays on an n270 CPU based netbook I use mpv and yt-dlp capped to 420p, even if I can play 720p@30FPS.

Good point. Though too late in this particular case, since the battery was also busted, I ended up e-wasting the machine.