My commute is 7.5 miles, with about 450-500 feet of climbing that is really just constant rollers rather than one or two big climbs. I run 46x17 (71 GI), which is a nice compromise between ease of climbing and descending. I have a couple descents that I easily hit 30+ mph on, which is about 150rpm cadence, but only for a few seconds. Any climbs below like 8% grade aren't a big deal, but once I hit about 10%+ (not on my commute, but pretty much unavoidable for me on any non-commute ride), that's where I start to struggle.

I'd say that on my commute and around the area I'm about as fast running fixed as I am on my road bike, but fixed I'm way faster than on my heavy CX commuter.