It's an oversimplification to think of the resistor as 'slowing down' the line, because that's not really what it's there for, at least in high-speed signalling, and it seems to imply that you would reduce or remove the resistor if you wanted to go faster.
In fact, it's series termination for the transmission line which the track represents. As such, its value, plus the output impedance of the driver, should equal the characteristic impedance of the track.
When your driver launches an edge down the line via the resistor, it travels down to the far end at half the final voltage (because there's a potential divider formed by the source impedance and the track impedance), and is then reflected at the open-circuit represented at the far end, which doubles its voltage to the full level. The reflection travels back to the source, at which point it is terminated by the source resistor (via the low impedance of the output drivers).
So the far end gets a nice clean edge, which it can safely use one propagation delay after it was sent (i.e. as soon as possible), and there isn't a set of reflections sloshing backwards and forwards for multiple round trip times, which causes EMI/crosstalk and delays.
The disadvantage is that if you look at the middle of the line, you'll see a funny stepped waveform, which means this isn't always suitable technique for multidrop links. (Certainly not multidrop clocks)
Just to clarify, it's the rise-time of your signal which matters most in these situations, not the frequency with which you generate edges. In an ideal world, you would always have drivers which had edge rates which were sensible for the frequency you were trying to transmit, but that's often not the case nowadays, and if your driver rise time is short, then you need to be thinking about ringing. On a data line, this might not matter (other than EMI), because it will all have stopped before the next clock edge, but on a clock it might be a double-clocking disaster, even if it's a disaster which happens only one million times a second.
Howard Johnson reckons you should be simulating anything longer than 1/6th of the rise-time to see if you need termination. At 1ns rise time that's 150ps, which is about an inch. Other people say things like 2 inches per nanosecond of rise time is the critical length for needing termination.