Shield speed has nothing to do with your weapon; your weapon speed is affected only by its speed rating, your wpf, and your armor weight penalty to wpf.
Shield speed affects the delay between when you press the right mouse button and when the shield is actively blocking. It's been said that 100+ speed shields block as fast as manual blocking, but that's clearly false (shields always block noticeably slower than weapons, especially within a short time after you drop your block).
In practice, it just means that if you carry a slow shield (<~85 speed), you can't "block feint" i.e. drop your block to entice someone to release a held attack, because you won't be able to raise your shield again in time. Also fighting multiple people becomes more difficult with a slow shield. In 1v1, you can compensate for a slow shield by just being conservative and blocking early.