No, speed increases are cumulative but do not exceed "Maximum!".

If you're using two Elite Quickstrike Trinkets and a medium ASI UV on a weapon (Medium + Medium + Medium = Maximum!) the speed increase low on your helmet is wasted.

There is an easy way to check this: if you open your knight's current loadout you'll have a few weapon slots to the right, but Under your knight are those same weapons listed too. If you mouse-over them, you can see the effect of all your gear's bonuses added up. If it says "Attack speed increase: Maximum!", try unequipping your helmet. If it still says "Attack speed increase: Maximum!" you know your helmet's bonus goes over the max and is essentially wasted.