Arduino Uno pin response speed
If you need pulses repeated at regular intervals, i.e. a pulse-width-modulated (PWM) signal, using the hardware timers on the ATmega328 might be the way to go. This gives you very precise and consistent timing and allows you to toggle a pin as fast as \$f_{CPU}/2\$ (8 MHz on an Arduino Uno), but it pretty much restricts your waveform to "high for X cycles out of Y".

This Arduino article has an overview of the topic.