For the bike, one thing I notice in shorter races is that it takes me a bit to get loose and in a rhythm.
I call this butt-lock and there is really no way to avoid it other than sandbagging the ride. You go from standing around, to swimming or running and then jump on a bike in a constricted position. Your body likes to warm up and you are not giving it one. I doubt if you are leaving anything on the table. You SHOULD be getting off the bike feeling like you have more in you–otherwise your run is going to suck.
From personal experience, longer-warm ups had no effect on race performance or butt-lock. One duathlon I rode for a good 30 minutes, ran for 15 and then went almost straight into the 5K opening run. The run was fine and I hopped on the bike and started to hammer. Yep, still had butt-lock. When you have a 20-40 minutes swim proceeding your ride, I suspect all warming up does is burn calories you need for later.
Doing a lot of bricks might help you get used to how it feels, but it won’t really make you faster or make that sensation go away.
Chad