Been running Parallels on my Mac for about 4 years on the same machine. I updated to Yosemite beta build 1 and was still using Parallels 9 with no issues. I upgraded to beta 2 yosemite that was released 10.10 and then decided to go ahead and upgrade to Parallels 10 the same hour. After that my virtual machine crawled to a halt. Booting time was taking ages for Windows. Programs were running extremely slow. No issues before all this. These are the things I have done so far: -Uninstalled parallels and re-installed -Deleted my virtual drive and re-installed -Increased my ram allowance Nothing worked. I even uninstalled everything, repaired my hard drive permissions under disk utility. Here is the kicker-I also installed VMWare and same issue. I also bought a brand new windows pro disk and installed it with vmware fusion-same slow performance. My mac is unaffected. No problems at all. Only on the virtual. I have no idea what else to do. Couple things to note: Tried to install parallels 10 again with new windows disk but parallels said I was not connected to internet which I thought was odd error and it prompted me to do offline activation. This is when I decided to try VMware instead. This all happened after the yosemite beta 2 so don't know if there is an issue there but I haven't seen anything about it. I was thinking about doing a complete fresh install of yosemite to see if that will fix the problem but wanted to see if you guys had any things I could try. I am at a loss at this point. The weird thing is the performance is just as bad with vmware and a brand new windows install disk and that my mac is unaffected.