THIS IS FYI:
Just switched to MC 8.5 (El Capitan) and it gave me this weird error that my AMA plugin panasonic P2 was incompatible, so I just ignored it.
I tried to render a simple 1 minute clip as an mp4 (h264) and it was saying it was gonna take more than 2 hours to render!! What?!
So, I deleted the AMA plugin from Library/Application Support/Avid/AVX2 plugins and everything is rendering at full speed!!
Just thought I'd share this in case anyone else is having this problem.
Happy editing!