I don't see Izotope, the real resource hogger. But it is what it is man, your system (combination of OS, DAW, Hardware etc...) has a beef with that plugin and that's it. I assume Mixbox is just T-Racks minimized so testing T-Racks may provide bad results too, or not.Good for you. But I have zero issues with Fabfilter, Softube, PA, UADx, Soundtoys, ToneProjects, Arturia, Liquid Sonics, Relab Acustica Audio. And know what, IK go to hell with the issues I have had - really bottom of the list amongst others.
But your OS isn't what Apple is providing support now, and they do not care at all, so please, consider that there's an issue with architecture that no one can provide solutions unless you upgrade to one Apple support knows.
Take the L in this, it's not the plugin.