It's been my experience that fast servos (I run HS5925s on 6V and they are *fast*) make a plane feel much more resonsive and "tighter" than slower servos.

Even when hovering, fast servos allow you to use large inputs of short duration -- which in some aircraft, make for more stable hovering.

Same goes for high-alpha rolling harriers, etc -- if your servos are slow it can be hard to get enough rudder/elevator deflection at the right time )with a small model) to allow sufficient control to do rolling horizontal figure-eights for example.

There have been more than a few occasions also, when having very fast servos have saved my plane (phew!).

However, for your average sport flying (as opposed to 3D) where control inputs are usually quite small, even a slow servo (0.23) will keep up with most people's thumbs.

Flying 3D though, often requires quite large and rapid inputs -- that's where fast servos shine.