Great game, bad coding
This has all the makings of a great game: good concept, decent graphics, addictive gameplay... it should deserve a 5/10.
But the game is buggy, whether or not you admit it. Most likely, the bug is in the code that deals with enemy spawing as the lag/framerate problem always occurs when new enemies are coming onto the field, whether it is the 2nd enemy or the 20th. It isn't the players machines, or their internet connection, or the host site... it's the code. Fix it and this game will get the score it deserves.