When I try to sort files by Filesize, it goes very slow with Preloading Metada, and starts all over again, when I switch focus to any other app, so, at the end, I dont get sorting at all
All of the custom sorts (the ones with numbers) are going to be very slow because they're done in the local contact sheet rather than on the database. You can, however, adjust your preferences so that rescanning isn't done when resuming work in Photo Mechanic after having switched out.
On the Contact Sheet page of the Preferences dialog, uncheck the "Rescan on resume" checkbox. You'll have to manually rescan when you have made changes to your filesystem when you were switched out of Photo Mechanic Plus.
-Kirk