AppleScript supports the "delay" command. System Events supports both "keydown" and "keyup" in addition to keystroke.

Apple Script is a programming language and you have full control flow at your disposal to wait and branch as needed.

This is too generic to answer this specific question. Key down, for example, is not an inherent property of a programming language.

(and neither is delay precision guarantees, which might be important for some key sequences)