Do you use macOS Mojave? In that case you might have to enable full disk access (see System Preferences > Security) so that the plugin for Apple Mail can be used. This speeds up importing/archiving a lot.
Yes, but when it is busy, it does not use more than 10% or so but it is reported as “not responding” because I asume you aren’t “respiring” the OS. As Windows developer myself, I’ve seen a lot of large for loops inside events instead of creating dedicated threads (or separate interface from implementation via some kind of Doc/Vista model) that completely locks the user interface and reports as “not responding” (sorry, Windows terminology, I don’t know macOS development ones).
Normally, without having to rebuild important parts of the application, that is resolved adding a call to Win32 Sleep() call. If you pass 0 you are telling to the OS that “do your things and give me control ASAP”. If you pass 1 that means you can give time to other threads in same application. More than 1 is a real “sleep” time.
(Sorry, Windows terminology a new time. I don’t know macOS equivalents).