Some iZotope plugins are very resource intensive, and if you put anything on every track, it's going to catch up with you sooner or later. I know that Neutron is something they tout as putting on every track, and TBH, I do stick it on many, but the only thing I use it for by the time I'm done is EQ, because I like the masking feature. Everything else goes by the wayside before I'm done, and I replace it with a stock compressor (usually that's all), and use aux tracks for reverbs.
In the little bit I've tried to use the latest version of GB on my ancient (2010) MacBook Pro, I've found it's more demanding than earlier versions, as well. I suspect it's the usual problem of the developers working on newer/faster machines and not really having requirements ("technical budgets") about how much CPU, memory, etc. they can use.
But, with a few tracks, if you limit your plugins to EQ and a compressor, you should be able to get by - even my MBP limps along Ok. (I don't have the iZotope suite installed on it - just an old version of Ozone 5.)
Incidentally, GB does implement an aux for delay/reverb, kind of behind the curtain, but when you have the "track" selected and adjust master echo or reverb, they're actually routing some of the signal to an aux.