According to the manufacturers specification, my current commuting bike weighs 11.5kg stock. Add pedals, rack, lights and the commuting cargo, and it's probably between 20 and 25 kg most days, although I've never been interested enough to weigh it. I could probably shave off a kilo or two, and I could go out and buy a super-light bike, but once I add my 120+ kg self to the equation the whole weight thing becomes a bit of an exercise in futility. I struggle a bit going up the hills, but have a lot of fun going down them.
In the end though, commuting isn't about speed (although thanks to all the commuting I am now quicker than I was), it's about getting to and from work. You can do that with an 8 kg bike or a 28 kg bike, the only difference is when you have to set off, and when you get back.