By raising the MTU, you can achieve faster data transfer (more data is sent at once), HOWEVER, if you raise it too high, you'll end up trying to push more data through the pipe than it can handle, causing dropped packets and lost data (which causes retransmissions).
Think of it this way:
Say you have a water line that can handle 1.5 gallons per second. Now, you start pushing 3 gallons per second. Chances are, the pipe will now start "dropping" or leaking water.
Of course, if you set your MTU too low, this can limit how much data is transferred in a certain amount of time.
MTU isn't the only thing that affects your data speed, however this FAQ does not cover this topic.