If you have a suspension fork, it's robbing you of (some of your) energy. You're pushing the shock absorbers around, and that power that you're generating isn't doing anything at all to move the bike forward. If you stand up on the pedals when you climb, you're putting a lot of work into flexing the stem. Plus, shocks are heavy. If yours can be locked, so that it "pretends to be" a normal fork, then that will help with the first issue, even if it won't make it weigh any less.