There is no direct correlation between Pi speed and power supply Ampere rating. However, if the Pi supply dips below the required voltage, the processor may not be able to run at full speed, may become erratic or may even re-boot.
Typically the smaller 5Volt supplies (say 1A and below) are designed to produce 5.0V by the regulator within the ‘black module’, which is connected to the device via a USB cable. There is a voltage drop from the regulator to the Pi which is proportional (approximately) to the current drawn by the Pi. This is usually the source of power problems.
Some of the 1A and larger power supplies are designed to provide a regulated voltage greater than 5.0Volts. For example, the Northpada 2A supply that I frequently use typically generates 5.25-5.30V. So the voltage measured on the Pi circuit board remains above 5.0V during normal operation with several USB peripherals connected, despite the voltage drop in the USB cable.
If you have a cheap multimeter you can measure the voltage on the Pi board and get a better idea whether the supply is suitable. Take a look at my old post for more details: https://captainbodgit.blogspot.co.uk/2014/06/powering-raspberrypi.html