I was investigating a phenomena of very slow selection of objects and an unintentional motion of object to the cursor position although mouse is not pressed (object not dragged).

I found, after a lot of test and eliminations that the problem was conflict with the app 'RightZoom' that allows Window Zoom while keeping menu visible, something that Mac normally does not do (menu line disappears, appearing only when mouse is pushed to the upper border). Once the app is shutdown or removed, selection goes back to normal - fast and stable.