Just to add my experiences thus far w/Parallels v15.1.4. I'm running it on an older 21.5" iMac (late 2013) w/a Quad-Core Intel Core i5 & 16 GB of memory. The only VM I'm running is Windows 10. I use Office 365 apps primarily under the Windows 10 VM, almost always Excel. After I resume Windows 10 after launching Parallels, it's typically around 5 -7 minutes before I can get a response after clicking the start button. From that point, I can launch Excel and use it, although there is some lag. This definitely seems to be an unacceptable performance decrease over v14.