Are all 64x64 px, the animation has four frames in loop and a 2.5 speed and is optimized.
Even creating five enemy at a time (when the first wave with 5 die, comes another wave with 5), play "stutters" from scratch.
I ran on the PC with the debug CS2, CPU usage is wavering, I found 80% too much for what was happening on the screen (5 enemies with bullet 90 degrees).