This reminds me of the time when I was looking up the best way to EMP and snipe fast with my ghosts, and I found that the best option seems to be just holding down the hotkey and left clicking where you wanna cast it, but I also heard that if you do this and click too fast (like, say, if you wanna snipe a templar so you double click snipe really fast on it) you might end up untargeting your ghosts instead. Not wanting to deal with that at the moment, I never tried to learn this method, so I haven't tested it myself, but it seems like this issue is related to the marine splitting issue.
What's causing this? Is there some mouse setting that will fix this, and if so is there a respective keyboard setting considering the stim command sometimes didn't register? Do I need a mouse and keyboard specialized to deal with such fast commands, and if so which specification should I look for? Or is it a fundamental limitation of the game that everyone has to deal with, and if so how do the pros do it?