Any new equipment automatically increases speed. I cut 3 minutes off my commute by getting a new water bottle. I think it was the insulation. Unfortunately I didn't screw the cap on tight enough one day and I lost it on the way home. Now I'm slow again.

Seriously though I've heard that new wheels is one of the few upgrades that can make a significant difference. In my case it was pretty slight. For awhile I kept my bike hanging from the rear wheel in the office. The bearings were definitely much better than my old wheels. I could spin that front wheel and it would go, go, go and go some more.