Edit curve's time scale

I'm working with a particle emitter which emits particles that go in circle. To do so, I used the velocity over lifetime property and set the x curve to cos(t) and y curve to -sin(t). With max velocity (y axis) set to 1.0 and timescale (x axis) set to 1.0 this gives me a circle of radius = 1 and speed = 1. To increase the speed however, I need to edit the timescale, or more precisely, the period of both trigonometric functions. I don't see how to do that, though, and I don't like the idea of changing those curves manually...

Thanks!