When you are running around attacking/blocking. The sprint speeds aren't that different. The combat speed is.
I call bullshit, I've been using a shield and I had no problems keeping up with people.
Sure they weigh quite a lot but then:
1. It's not "dead" weight as heavy shield means thrust stun galore for the opponent
2. If you want to move quicker you can just grab a lighter shield (Elite cav is my fav, although just a regular cheap-ass round cav shield will hold up enough if you put it away when not needed) and tap-block.
It's very effective.
If you hold down that rmb you are going to move slower, but then that is true for all weapons.
EDIT: Vibe, I think it's ~17% for blocking and ~50% for backpeddaling.