I use Melodyne as an ARA extension all the time (on Windows 10 with about a 14-year-old system, so decidedly not a powerful modern CPU) with no annoying issues of the type you mention. BUT I’m not trying to do other sorts of edits on clips with Melodyne (or other ARA extensions) active on them. I use ARA extensions for editing and commit those edits prior to doing further operations. Or, alternately, I’ve done those other operations, especially comping and trimming fades and bouncing the comps, prior to using Melodyne (which I pretty much only use for tuning – I use VocAlign for tightening parts).
I don’t know much about ARA internals, but I think that, when you modify something inside an ARA extension, there has to be some caching of the modified audio, which would be what gets played back. If you’re tweaking that, then the cache probably needs to be regenerated. And, if you’re making external edits to the clips, that probably also changes what needs to be cached. If I’m correct in my thinking on this, it may be that regenerating the cache after changes makes things sluggish, especially in cases where clip length is changed, which probably results in needing Melodyne to process more or less information than it had been doing previously (and possibly analyzing any information added).
While I know it is possible to use Melodyne while comping, I never do, as it just wouldn’t be efficient workflow for me since I tend to have lots of takes, which I’m cutting up into phrase-level clips for auditioning purposes. (I’m also often comping multiple parts from the same pool of takes, for example to double a background vocal part.)
Because I do want to consider tuning, to a degree at least, while comping, I tend to use another tuning plugin (usually Waves Tune RT these days), either set for a chromatic scale or the scale of the song, while comping. That way I’ll know if things are at least in the ballpark (or if the “tuning” actually makes things worse by sending notes in the wrong direction). Then once I’ve done my comping, I do any audio clip editing (e.g. trimming, fades, possibly clip volume adjustments) and bounce the individual song sections for each part. It is after this point that I’ll use Melodyne, after removing the temporary tuning plugin, to do its thing. (On background vocals or lead vocal doubles, I’ll generally have also used VocAlign to tighten the parts prior to tuning.)
In general, I never leave ARA extensions inside a saved project file, even in disabled tracks. Any backups I have of tracks prior to bouncing the Melodyned clips will be prior to having used Melodyne, and, if I need to revisit them, I’ll just cut out the specific phrases that need to be revisited and do the Melodyne work on them again.
Oh, one other thing that I have noticed with ARA extensions that may be relevant, though different extensions may work in different ways. At one point, while using Synchro Arts RePitch, I tried using the setup option that saved the ARA data inside the Cubase project. (I was thinking it just meant it would save it in the ARA folder under the Cubase project folder, instead of in a system-wide location, but it actually saved it in the CPR file.) When I was doing that, I noticed that I’d get long pauses every time Cubase’s automated project backup kicked in. The reason was that the size of the CPR file was growing significantly due to the added ARA cache data (basically audio). It was so annoying that I reverted to its saving the cache data on disk, despite my not being happy that it was in another location I’d have to manually check to clean up at some point. I’ve have not seen an issue like this in Melodyne, so it probably is not saving its cache inside the CPR file.
