I am seeing exactly the same problems as @Michael59 on a 5K iMac that runs El Capitan. I tried every KB article tip that has been referenced for this and other slow VM issues, but none of it has made the slightest difference. I increased the video memory for the Yosemite guest to 512 MB, and that helped a little. By comparison, I have a Mavericks VM with just 32 MB of video memory, and it runs like a champ (and has since the day I created it over a year ago). I have other guest operating systems (Ubuntu, Windows 7 & 10) that work without any issue whatsoever.
I have been using Parallels Desktop for a long, long time, and this is the first time I have ever seen something like this. It makes me think I did something wrong in creating the VM simply because this software has never let me down before.