While I might not have empirical data to back it up, experience says that these are intensive operations regardless of the front-end technology used. This is primarily due to the fact that you are relying on the user's cpu cores to do this work. This isn't to say that one is better than the other - it's more that the ROI for a re-write probably will not pay off like you expect it to, based purely on performance reasons.
As for single-threadedness - the Flash Player Platform supports workers, which actually sounds like a good candidate for your application as it can more easily be divided into discrete cores.
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/system/Worker.html