I Have a problem when I'm in combat with the drones, when I'm in combat my frames drop down to 10 frames (ussally around 30) and when I escape the drones are still in the background they keep my fps very low. So would you mind just placing one drone per attack and once you get too far away from them they should just disappear.
In 0.8.4 there is a option in menu that sets the count of drones per wave. Also I added a weapon that disables drones in case you don't want to fight one.
0.8.4 is coming today/tomorrow.
What specs do you have? even on my cheap potato laptop I was getting 60 fps in combat, I think the issue is not with the game but with the graphics library, do you have directX10 or 11 or OpenGL latest version? also what OS do you have?