Well, see e.g. https://blog.semantiscope.com/2009/10/29/getting-pointers-fr... ; the Flash runtime relied in various places on how pointers are represented for various "smart tricks"/optimizations. While this may have had the local effect of being faster, it also had the side effect of making global optimizations in the VM harder, since the VM was limited in the ways it could change. With the flash runtime codebase being pretty large, and the team being fairly paranoid about backwards compatibility ("you don't want to be the one who breaks the internet!"), this slowed down change quite a bit. Sure, it was easy to add new features - but it was pretty hard to improve how the old ones were working.