From memory a 486DX4-100 or an overclocked 486DX2-66 (to 80MHz) could just about handle 128kbit MP3s

Ah nostalgia... This was my first PC processor. I remember my CPU came with some sort of a bug that prevented me running windows 3.11 on it.

Yeah, I seem to remember there was that brief time period where my PC could barely play MP3s and I used to treat them like zip archives -- rendering them back out to WAV files to play them back.

I think you're right and that was around the 486 DX2, DX4, Pentium transitional time period.

Interesting. My Amiga with a 68030 @42Mhz could just about pull off mono 128Kbps decoding (as long as you didn't multitask too much). That would suggest the 486 was almost clock comparable to a 68030 at least for lossy decompression, perhaps the 486 was around ~110% had their been a 68030 series 80Mhz equivalent.

I wonder if that was also an I/O limitation rather than just a CPU thing. Old floppies probably would not handle 128kbit. Hard drives were slow too, so was memory and bus.

I remember I could play 128kbit MP3s in full quality using mpg123 running under Windows for Workgroups 3.11 ONLY when I overclocked my 486DX4-100 to 120Mhz.

I know a 486 can just barely play them (while doing absolutely nothing else)

Even Pentium class CPUs had problems handling MP3s depending on the encoding

Doubtful. Back in the day when my 'work computer' was a Pentium (Pentium 200MMX to be exact) playing a MP3 consumed 50% CPU. Note, I no longer remember if mpg123 was using MMX instructions or not.

Given how much faster a Pentium 200MMX was than any 386 CPU, I very much doubt a 386 (even a DX) would have had enough power to decode in real time a MP3.

I remember using winamp on a Pentium 90 and it was up to the job single tasking. I couldn't say the same about the one time I tried to encode.

I can't recall what hardware I first used an MMX-optimized encoder. I remember it making a huge difference though.

Found this (cubic player on a 386DX at 40MHz) https://www.youtube.com/watch?v=6Jj97NXgHw4 and I'm wondering how much a 32bit ISA is better than a 16bit one for integer decoding.

I used to listen to mp3s on my FPU-less Sharp Zaurus clamshell using madplay [0] with cycles to spare.

Linked from [0] is a page claiming to use MAD for mp3 playback on a Newton [1].

Not sure how those compare to a 386SX, but it's probably a good place to start.

[0] https://www.underbit.com/products/mad/

[1] https://40hz.org/Pages/newton/packages/madnewton/