What exercises or training can i do to improve my speed when i do karate?

More karate: As with most life activities, increased time spent engaged in the activity will increase the proficiency with which one performs the activity. This is sometimes colloquially referred to as "muscle memory".

Master your basics: Train at different speeds when practicing your techniques. Emphasize knowing your basics. When you approach your training that way, with practice your techniques are no longer a series of moves; rather, they become your reflex and reaction to situations.

Train for power: You should be training for power/explosiveness. Usually this is done with olympic lifts like the clean and snatch. Plyometics can be helpful.