Google has added the +1 option to all their search requests / results, I'm sure if the performance was such an issue they would of found a way to fix it as it would degrade their page load times, granted I'm sure it's not the same 'exact' code.
Edit: you can. This example [1] has opacity:0.3 so you can still see it. It seems as though it would be trivial for an attacker to put the +1 button over every link on a page. You may even be able to have one tracking the mouse cursor.