Yeah, except at the extremes, your gearing doesn't dictate your speed, since you should be able to compensate with cadence. And I find that the harder I'm working, the higher I want my cadence to be anyhow.
On level ground, there are three things slowing you down. In order, they are: aerodynamics, rolling resistance (tires) and mechanical resistance (drivetrain). Bike drivetrains are extremely efficient, so there's not a lot to optimize there. There's a fair amount of room for improvement with tires. The stock tires on most bikes are generally not very good, and you could possibly save 30 watts by getting high-end tires. Your bike puts you in an un-aerodynamic position, and there's not a lot you can do about that economically. On hills, of course, gravity comes into play.
But all other things being equal, power output dictates speed. You'll need to work on the engine if you want to go faster. Also bear in mind that in urban commuting, it can be really hard to change your door-to-door commute time when you're waiting at stoplights—you need to increase your speed enough that you're beating lights that you couldn't beat before. Otherwise you're just sprinting to a light and waiting at it longer, so your net time is the same.