I did a couple gens with a +3 Heavy Round Shield (87 speed), then switched to a Brown Lion Heater (96 speed, mine is +2 now, so 98 speed).
The difference, as far as I can tell, is that with the 87 speed shield, if I fall for a feint, I can't recover fast enough to block. Whereas with the 98 speed shield, if I fall for a feint and drop my block, it's no big deal, I just block again before they hit me.
The devs should add more shields in the 94-96 speed range (like the Brown Lion). They loom into being near max speed, but don't pay a hitpoint premium for their speed like the Knightly/Elite Cav.