I tried clearing the website data and loading and letting it sit for a couple minutes, and I'm sorry to say the performance got worse, hovering at about 1-3 fps. :(
I still think it might be related to the landscape problem I mentioned in the other thread because in both cases, the fps is at 60 until a bunch of sprites get created - even though that problem only happens in landscape and not portrait, whereas the space blaster problem happens in both modes.
I really don't know, just trying to come up with ideas. For reference though, I don't know if it's a power saving setting or something about the warmup period, but I've never encountered it on my iPad when doing performance tests - and in the landscape performance problem in the other thread, that's definitely not the case because the performance difference is instant when switching back and forth between landscape and portrait.