serial versus parallel bus
On a drawing board, parallel look faster than serial. At least, this is what we were told since our basic electronic courses.
However, there is more than drawing paper, i.e. actual physics involved.
At higher frequencies, you gat a whole lot of new constraints. For example, you no longer can use 0V/5V signals. This is why higher frequency bus go to 3.3V and even lower.
A little theory here... However, going lower voltage meen that the bus is more sensible to noise. Moreover, the longer the data lines go, the higher the noise level. A way to counter-interact this is to use what's called 'differential lines'. A differential line is simply a pair of lines which are tightly coupled (run side by side for example). Then, the driver always put a logical 0 or 1 on one line, and the inverse on the other line. For example, it would put 0.5V on one line while putting 0V on the other line (to represent a logical 1), or 0V on one line and 0.5V on the other line (to represent a logical 0). The theory is that if noise occur, which will result in voltage spike on the lines, since both lines run side by side, that voltage spike will occur simultaneously on both lines. At the receiving end, even if the lines are noisy, the *difference* of voltage across the two lines of the pair will be the same, and reflect the transmitted data. So, the receiving end just look for this voltage difference, and extract a logical 0 or 1 depending on which line have higher voltage. The good thing is that you don't require a common refferene between the transmitter and receiver (like a ground). So, even if the transmitting device and receiving device have different GND (due to noise for example...), this doesn't affect the transmission. Differential voltage pair are used in many applications, like Ethernet (twisted pair cables).
So, due to the above reason, serial links can go much higher frequency that parallel links. And at 2GHz+ frequencies, even a 2 inch trace on a PCB benefit from the above concept.