Well they did lag, but very little.
I didn't hate the music, and I kinda think it fits the game well. Yes I like blood and gore. I think it's more of a "Click as much and fast as you can and then aim when there are few flies left!"
I think you should add upgrades, boss flies and stuff. The upgrading is because you say we get money(not rl money ofcourse) from killing red and blue flies.
So, I think variety is a option, like the above mentioned and maybe like a single fly that flies fast or some flies that stay at one place, but timed so that it gives challenge.
Also, you should add a replay and mute button, lol.