Sounds as the same problem as with SSL and Arturia plugins. Everything works fine, untill you open (and close) the gui, than everything starts lagging, rendering super important vst’s unusable in Renoise.
In a couple of hours somebody will post here that it’s a problem that needs to be fixed by the plugin company, but it feels like the same cause. I find it strange that these plugins seem to work fine in every other DAW except Renoise. I can’t imagine that these plugin builders have if statements in their code for every daw to get the thing working in all DAWs, then again, what do I know 
It’s the exact seem behaviour popping up only in Renoise, from plugins built by several prominent plugin companies, that has to mean a bit work for Renoise too.
I hope this gets fixed magically someday for you and me, I really miss my SSL plugins.