Finally, after some 7 weeks this is solved!
It turned out to be Arturia V-coll instruments after all (which I always believed.)
After plenty of support cases both with Arturia and Ableton, Arturia finally suggested I could turn on logging from the plugin instruments and send them the file. So I did. This is done in /Library/Arturia/<instrument>/resources/log.conf
(a bit cumbersome that you have it turn it on for every one of the instruments, but I guess one or two would have given an indication anyhow).
By simply removing the double slashed before the two lines:
this produces a log file "logArturia" on the desktop.
In the log file I found this warning: "Categories invalid, refresh instrument id on database."
To me this sounded very suspicious and it turned out is was.
Arturia the asked my to update the "preset index" by deleting a specific temp file called "db.db3".
And this fixed the problem. No more lagging in the GUI and no more slow exports.
Arturia explained something must have gone wrong when upgrading the Arturia V-coll instruments. Just as I thought.
Anyhow. Happy ending!
Hope this can help somebody else...