this isn't a construct2 bug. It's the browser's rendering engine and HTML5 support.
Since the last window release of safari (5.1.7) the mac version have been getting improved HTML5 performance.
Opera is using the Presto engine as of Opera 12.15. If you go play other HTML5 games not even made by construct2 you can see how sluggish it is. Opera announced they are switching to webkit which is what chrome is using until they announced to use Blink which Opera is going to as well.