As I understand it, the AR15's got a relatively long lock time - wouldn't a high speed trigger (e.g. Geissele) offer a real benefit when target-grade accuracy matters? Is it standard equipment in highpower competition, for instance? Just curious.
The stock AR trigger does needs work to break clean. Which I think takes precedence over lock time. Once done the rifle can still be equal to faster systems. That said, I have experienced shots I gave up on because of poor technique that were saved by Geissele's faster lock time.
But great scores have been fired with slow systems which is why I say fundamentals and technique are more important. In the end it is the 'nut behind the trigger' that's responsible.
As to faster lock times the hammer or striker is usually a lighter weight material. As such they need stronger springs to drive positively into the primer. Many ,like the Geissele, have completely different throws and sear geometry from the stock assemblies. They can be more fragile in that more attention may be needed to keep them operating properly. Falling out of adjustment is an occasional malady.