I noticed a slight delay with my USB convertor when I used a Sony Dual Shock psx controller, but when I use my cheap Gamestop digital controller, I have 0 problems with delayed fireballs. I'm not sure if the brand has something to do with this, or it could even be the drivers, but trying a 5 dollar cheap controller might be worth a shot to see if that's the problem.
-CthulhuLuke