Do you have Continuously Rasterize set for the Illustrator layer? There is a known issue with layers based on Illustrator files if they aren't continuously rasterized. You can work around the issue by either turning off Render Multiple Frames Simultaneously multiprocessing or setting the layer to Continuously Rasterize.
That was it!
My AI file was in a sub-comp but did not have continuous rasterise checked. I turned that on in the base comp and it rippled through to the master comp. Multi processing now works as advertised! 4-6 minute renders down from over 20-30 minute renders.
A time saver....yes
Thanks Todd, I may switch to Premiere if Adobe ever get savable transition with customisable gradient wipes for lower thirds.