Having problems with a 32 core Xeon as well and of course the scenes are such that I can't share them. Maybe I get the time to reproduce the problem on spare time but the issue is, sometimes the first pass renders for like 30 minutes. Also it slows down during the estimated time remaining. Exterior scenes using some forest objects and displacement, I have 128 GB RAM on the machine and plenty of space on all drives, Corona using about 20 GB IIRC, so it shouldn't be a memoery issue. But no matter what I do only one core is rendering while the others stand idle.
Also, maybe it's just me but I just seem to find the images are more.. blurry? lately. I'm using pretty much standard settings, only displacement precision changed to 1. Also tried with adaptivity on/off, no difference. 2 light layers, sun and sky. Haven't touched the reduce noise filtering, either.
Is this something you're about to nail down, maybe? :P I'll be sure to send a scene if I get to reproduce it on spare time, once I have time and enough energy to just play with everything until the rendering process begins to fail.