Come on, Volker, restarting kmail shouldn't be such a problem!
:) Depends. I'm also wiping all its index files because kmail is unable to keep them matching reality, so startup takes time (as it also indexes absolutely everything, needed or not, on startup).
The alternative of constantly rescanning the ~/Mail/ folder content cannot be good, especially when we talk about many subfolders ...
That alternative is indeed bad. However, you'd only need to rescan a directory for either subdirectories or mbox files, you would only need to rescan the directory currently displayed in the index on the left (usually ~/Mail), and you would only need to rescan when there's activity in kmail, e.g. user changes folders. I make that a comparatively trivial amount of CPU time, certainly compared with the mindless activity to have to quit out of an app to make the app do its job (wasn't this a M$ solution?).