slow encoding from 50% to 75%

I export a 10 minute multitrack soundfile to mp3 at least once a day.  As it encodes, it gives the percentage finished and the time remaining. Without fail, the encoding proceeds rapidly up to 50%, and then goes REALLY SLOWLY up to 75%, and then finishes quickly. I can't figure this out.

I am using a 12" 2017 MacBook, MacOS 10.15.6, Audition 13.0.8 (all latest updates). With the Media Encoder, I switched it over to use "all software" encoding, since that was faster than allowing it to use "hardware acceleration" (which was the suggested setting), though I cannot find this setting in Audition.