The Lag is strange, it does not always happen.
But I think one way to help is to stop the clusters of enemies hitting the sides I think the texture over lap could be causing issues.
On one of my tests on my game if you have 4+ layers even if there separate images and Sprites. It can cause lock ups.
I'm not sure how to fix it.
I'm no where near as advanced as you. I'm struggling to make Tic Tac Toe lol
But I had 3 counters in one space and I got similar lag to what I am seeing in your game.
I'm not sure how to fix it. I'm extremely new to AGKS and Coding/Programming
But it's just something I wanted to point out to you.
If there is a new build out. Feel free to tag me on the Discord and I will help test. I like Tring to find faults and Bugs