For reference: I had a 486 DX2 66 MHz, and I played MP3s by decoding them in batch to PCM and then playing that as it couldn't really do that in real time. Perhaps only if you didn't do anything else and the MP3 was low bitrate enough and ideally mono.

MP3 decoders have, thankfully, gotten faster since the 90s. But yes, a 66MHz is pushing it indeed.