MFR slows down to 1 frame after some time

I have a strange situation where MFR will start out happily, renders about 20 concurrent frames for a while, then it slows down to 1 frame after about an hour or two of rendering. I have set it to purge memory every 1500 frames, and it doesn't seem to be hitting any out-of-memory errors. Once I kill the render and re-launch AE, the render starts where it left off at 20 concurrent frames, then slows down again. 

CPU will start out at 25-50%, then drags down to a crawl. 

The comps do use effects, none have the yellow warning icon. Effects used are time:echo and distort:turbulent displace. 

Output is JPG sequence.