Thanks for the feedback!
Could you explain a bit more about where the lag happens? Is it during a specific part of the game, or is the performance generally slow throughout the entire gameplay? That would really help me narrow it down.
Also, just to make sure, are you stuck at Level 4 because there are no upgrades left to buy? I'm already working on improving the XP progression for Level 5, but I just want to confirm that's the issue you're running into.