I used WinAmp for this in the past; it converted my midis to wavs, then another prog (dbpoweramp I think it was called), converted the wavs to mp3s. Two stage process, not speedy, but free and mostly hands-off operation.
I think current versions still support it, or at least the portable version did last time I tried it. If it's like it used to be, you have to fiddle with the settings a bit (setting output to be a midi plugin, or some such thing), but once it's set, you're golden. Set the output directory, shove all the titles into a playlist, and let it 'play' them all into wav's.
It's realtime, so it can't compete with the speeds mentioned above unless WinAmp has added some new capabilities to the plugin or whatever it was.
YMMV; as I recall, WinAmp was also very quirky and a wee bit buggy, so I did have to occasionally adjust the playlists to skip problem midi's that crashed it for some reason.