I tried running a few levels of Mega Mash to practice some runs later, but for some reason around level 2-3 the game starts slowing down really badly even in standalone. Not sure if that's an issue for anyone else.

I just tested the game in standalone and I had the same issue. When I ran the game in the past, I used Supernova which ran smoothly. Since supernova isn't viewed as a good alternative, using Flashpoint or using a browser that still supports flash may work.

Yeah even trying Supernova on my end it was still pretty slow. I did have some success using the Linux flashplayer but that started running slow at level 5, might just be something in the game itself but I have no idea what.