Hmm.
Morph, if you have any specific details of this happening I’d really like to hear about it (as would TVEyes I suspect!
)
In my experience, it’s not the number of plugins you have in the ZPlugs folder that causes ZBrush to slow load, “freeze” or simply shut down when booting up. Rather it’s a problem with scripts that contain syntax errors (I know I’ve written my share during script development.) 
Fixing the syntax makes the loading problem go away - of course you must have the .txt script to do the fixing - if you only have the .zsc version of the plugin all you can do is remove it from the ZPlugs folder.
I suppose there may be some fatal combination of scripts that used individually wouldn’t cause a problem (memory conflicts for instance) though I have yet to experience that. Aurick has hinted that conflicting plugins can cause bootup problems though I don’t remember him ever naming specific problem scripts.
Once or twice I’ve had to correct released scripts that worked on some systems but not on others because of differences in how some systems were configured. There are also some scripts that will run on PC but not the Mac or vice versa, especially now with the release of 3.12.
Ultimately, you can isolate the problem script through a process of removing scripts one by one from the ZPlugs folder, then restarting ZBrush.
That’s my 2 cents anyway, Sven