Servos have their own speed limits, typically expressed as 60 deg/ X seconds of time. The lower X is, the faster the servo can go. Different types of servos have different speed maximums.
To command the servo to go as fast as possible, just keep increasing/decreasing the pulse until it makes no speed difference.
The applied load also affects speed, so lower loads will equate to higher speeds.
If you require faster, you will have to buy a faster servo . . .