Hi Georg,
I had something similar in v1.6.0 - where even changing an image identifier caused PI to go into "wait a bit, I'm doing it" mode for a few seconds each time (Vista64). Again, this was resolved by rebooting PI - but I tend to only reboot PI when I have to reboot my PC, and even that doesn't sometimes happen for MONTHS (as I have said in the past, my darling wife thinks that my operating system IS actually PixInsight, and that PI is clever enough to do all the OTHER things that her PC does, like email, the internet, etc.)
However, I haven't encountered your 'slow-down', yet, since changing to v1.6.1