ZZ is only 3 keystrokes as opposed to 4 for :x (shift + z + z = 3 and shift + ; + x + enter = 4). It also doesn't require timing the release of shift before the press of x.
Wow! Never knew that existed. If I can undo 20 years of muscle memory, I'll use it. :D
EDIT: Just tried it, and Z being right next to Shift is a deal breaker.
Switch to a German QWERTZ keyboard. That’ll also help with giving rusty muscle memory a fresh kick.