CFL bulbs are not dimmable and usually take up to 30 seconds to get to full illumination. You can purchase LED bulbs which can be dimmable. Check the label carefully. The main problem with dimming LEDs is with the dimmer itself. Many of the regular incandescent dimmers on the market may not dim LEDs smoothly or fully, often resulting in a jerky or flickering effect. Specialized LED dimmers (LUTRON etc) will usually solve this problem.