There were no known changes that could have materially affected performance in recent versions. I'll push back a bit and ask whether you've added some data recently to your myplaces (either large 3D geometry, collections of image overlays, or some large collection of placemarks) that could have caused the reduced performance. A simple way to test it is to back up your myplaces.kml file (look at it's size) and then delete the original. You should fall back to a 19-placemark copy of default_myplaces.kml which should have no history at all.