Encode Slowed Considerably at Halfway Point

I'm encoding a very large video. 8 hours at 4K with several video layers and some effects. I do this every week and it usually takes about 36 hours to encode.

This week the original qoute from Media Encoder was 32 hours (it usually takes a lot less time than ME predicts) and it powered through about 40% of the encode in the first 12 hours. I could hear my machine roaring along as always. The next morning I noticed my machine was quiet and thought it had finished but when I checked it wasn't even at 50% yet and in the last 24 hours has only gotten about 5% further.

The video is 30 minute loop and all the effects are constant throughout so it's not like the content required heavier encoding part way in.

Activity Monitor shows that 83% of my CPU is idle and 75gigs of my RAM is being used.

Why did my encode slow to a trickle after the first 12 or so hours?

Machine Specs:

iMac (Retina 5k 27in 2020)

Monterey 12.3

3.6GHz 10-Core i9

Ram 128gb

Video Card AMD Radeon Pro 5700 XT 16gb