Original clips are MP4 avch. It appears that when Premiere is rendering and hits a long clip it gets all wound up and goes into sulk mode. But give it a lot of short clips and its fine; sails along fine.
The clips have colour changes etc mainly through Lumetri.
Whar I have been doing lately is to highlight a long clip, render it and then move to next long one. Once all long ones are done, I just tel it to render whole sequence.