I had the same issue. At some point after I updated to v15 (or perhaps after updating host Mac OS, can't say for sure), Windows guest machine started to behave "sluggish": from time to time any operation could take quite a long time, then there would be a period of everything working fine, and then sluggish again. At the same time, Linux guest machine was working absolutely fine. I tried allocating only one processor, I tried switching to Apple hypervisor - nothing helped. But then I saw a recommendation here in the thread to add Parallels machines folder to exclusions of Spotlight (Privacy tab), and after that the issue was gone! So I am a happy camper now, big thanks to those who shared this recommendation.
Dear all, This worked indeed >>> Disable Spotlight searching the Parallels VM directory at the Privacy tab. Many thanks to the users (!!!) who found this. Theo