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While following your advice I started first of all making sure that the display settings were right
They were (at least in theory) but anyway I set them again (I work with two displays, a main one and a cheaper one for the tools).
Well, MIRACLE!
I'm not aware of having changed anything but the fact is that not only (at least for the moment
) the problem disappeared but LR seems to work noticeably faster now.
I'm keeping your notes "just in case" the magic should reverse
You put me in the right path. Thank you