For some reason, Samsung has a type of “key-press timer” which forces you to slow down why keying anything in on the physical keyboard. Supposedly this helps reduce typos due to multiple keys being pressed at the same time — unfortunately, doing so means means you have to type slower, which defeats the purpose of the physical keyboard.
Luckily, the XDA community has a patch that “optimizes” the key timer such that keystrokes aren’t dropped as they are by default. You will need to have superuser permissions on your device to apply the patch, but reports are very positive!
Head over to the thread on XDA-Developers for full instructions and the necessary file.