Playing again it appears that the lag happens every time the enemy shots, so it may be because so many bullets are spawning at once?
Alright I uploaded a more optimized build and tested it on safari. Safari still has some visual issues and its not as smooth as firefox/chrome, but hope its more playable for you! Perhaps Post-Jam I will work on some more optimizations but with 20 minutes left I dont think there is much more I can do haha