Have you seen this thread?: Cubase 11 ARA folder is gigantic from using Melodyne
At least as it relates to massive Cubase slow down, it could be related to comped tracks. My take-away from that thread is to do any comping first and bouncing to a new track prior to adding Melodyne as an extension. I’ve had fewer problems using this workflow. It’s a pain because sometimes I might realize later on that I had a better take and it’s a hassle to get where I need to be.