Delay issue: Unity Game Preview vs Mobile Phone Device

When I'm testing my app with the Unity Game Preview tab, there's no single delay when pressing the fire key (Mouse 1 in this case). However, when playing the game on a mobile phone.. There is a short delay which frustrates me. I've got a couple of testers who do not experience this delay issue, but I do because I know better in this case. When I'm using Blue Stacks and install my apk on that platform (it's an android emulator), I have a delay as well. When I'm building my game for Windows, there's no delay.

So.. this is pretty weird! Is this normal, or does it have to do with my script? I'm using the update void and detecting if the fire1 key has been pressed:

if ( Input.GetButtonDown("Fire1"))