I don't know how large your set is but I have about 50 projects loaded into icomoon and each has at least 100 icons in it and i have not had any issues with speed. I love the app and it's one of the apps that I can't do any work without.

Also I had once accidentally deleted a project, contacted them and they were able to recover it within a few hours after my request. They are superb

I loaded streamline font set into it and it's actually close to unusable on my machine.

I'm glad to hear it works for others though.