Thanks for your feedback! I’m glad you enjoy the game :) What’s your current rank?
There’s a slight slowdown after closing the upgrade menu, but I’m planning to add a short freeze and a sound for resuming game.
Each update will improve the balancing and also improve the wave system! It's interesting how important such little tweeks are!. The next update will include a completely overhauled upgrade system with probabilities, rarities, and rewards for Exp/Level streaks.