Whenever I save an attachment, one CPU core is at 100% load, while the save dialog is open.
The same thing happens when saving an existing mail itself to a file.
(it does not matter by which of the many menus the dialog is invoked)

HOWEVER, only the dialogs in this narrow context seem to be affected.
"Open message in new window" -> File -> Save as -> File: still affected
"New message" -> File -> Save as -> File: not affected
Also, all other Open/Save dialogs I could find are not affected.

As it depends a lot on the context in which a dialog is used, it indicates to me that neither the dialog itself is the problem, nor something it tries to load (like a blocking netshare, but I don't have any network shares anyway)
It should be noted that I get native KDE Plasma file dialogs

I could imagine there's a problem in the way (parameters) those specific dialogs are opened.