As the title says my Windows 10 VM RAM has a max clock of 667MHz which is obviously making it run really badly. I've been trying to tackle this one for a while trying different combinations of memory settings and I noticed all the questions on the forum related to this have gone unanswered. To add insult to injury the support chat people have no idea what they're doing and just keep telling me to increase the amount of RAM allocated. Finally I found a solution that works for me. Change the hypervisor to the Apple hypervisor (Screenshot attached). There seems to be some weird bug or limitation in the Parallels hypervisor that creates this artificial limit on the RAM speed but the Apple one doesn't have that issue. I hope this helps you, if it did or you have any further insight into this please leave a comment.