They speed me up in general, especially on the uphills, rough downhills, and stream crossings. But there are some cases where they slow me down. If I'm REALLY trying to make time on the downhills, I can go faster without them, but it'll wear me out (both structurally and cardiovascularly). They slow me down any time I need to take the straps off/on--crossing gnarly blowdowns, taking a snapshot, boulder hopping, etc.
Overall, though, they allow me to go farther and faster with less fatigue and body aches.