Sending data over the serial port is extremely CPU intensive.
How many bits of data would be getting sent per second?
The MIDI transfer rate is 31.25Kbps, 10 bits per byte (7 data bits + 1 status/data flag + start and stop bits.) Each event consists of one to three bytes. The number of events per second depends on the song, but Wolf3D's music isn't terribly intensive.
(Just spitballing, but a song at 120bpm = 2b/s, or a probable maximum of 8 notes/second if it's all 16th notes - unlikely - and no more than eleven channels for OPL2 music if it's in rhythm mode. That would be a total of maybe 528 bytes for an unusually high number of note events, plus maybe some controller changes - so we'll say 640 bytes. That's a measly 6400 bits/second.)
But it won't lighten the cpu load compared to playing the sound samples with Paula.
Playing samples with Paula is no CPU load;
mixing audio (which you'd have to do in order to play MIDI music of any complexity along with sound effects) can be quite a lot, depending on the quality.