Hi 20021128!
Stages have been discovered to be the biggest memory hog in the current build. As of 0.7.1, we have optimized enemies to unload when off-screen, and their ai scripts have had their calls reduced per second. You may be able to play the game if you try the debug stage, as it's smaller.
We're planning to unload stages in chunks to solve the freezing issue, that change should be coming soon!