Hey Firebelley, thanks for checking out my game and I'm glad you enjoyed it!
Just to answer some of your questions from the stream:
"I would prefer to have a definitive end to the game rather than just survive as long as possible" - I hadn't considered this at the time, but in hindsight that probably would have been a lot better because a lot of people had a similar issue of snowballing into an unkillable lag machine by the end.
"It would be cool to be able to rotate the ship" - This was one of the first things I implemented into the game, but when playtesting it, it just felt a bit janky and difficult to control. I also figured it'd be better for a game jam to keep the controls as simple as possible. But this could certainly be explored again in the future.
"Maybe the way you're trying to find the nearest enemy for each turret could be causing the lag" - This is almost certainly the biggest contributor to the lag, because you're were right in that I was doing it as inefficiently as possible haha. Your suggestion about using the collision system is really helpful, I'll have to give it a shot.