Thank you so much for the, in depth, analysis and feedback. It is hugely appreciated!
The performance issues are very much at the top of the priority list and, while we're still hard at work on our other project, this will be something I'll tackle as soon as we return to this game.
There are many, many improvements still to be made when it comes to performance: Object pooling, reduce object count as the horde gets bigger (like the blue glow you mention in your testing), sound optimization when the horde gets bigger, etc.
However, from my testing, the biggest culprit is the current code that handles zombie/cured collision detection. As the game goes on, a number of zombies/cured get stuck around the map or begin "clumping" significantly which causes the unoptimized code to take way too long per frame.
I absolutely cannot wait to return to this project and, after dealing with the performance issues, have some good fun adding a ton of features we currently have on our plan. Meta Progression, unique zombie types, more upgrades, ability upgrades such as a dash or teleport, equipment upgrades that modify how you apply the cure, more maps... It's gonna be a fun time.
Thank you again for your comments, you're an absolute legend! <3