Many times songs are so fast that it’s necessary to only play down strokes (like an eighth note groove) for a particular song.
The only thing to watch out for is how tense your hand is. Your strumming hand should always be relaxed. I imagine that “super fast” and “relaxed” don’t normally exist in the same sentence, so this might be more of a mental exercise than a physical one. Also, make sure you’re not hinging at the elbow, as this could ad to the tension in your wrist. Your wrist will tell you when it needs a break. When it does, listen to it!
The correct wrist movement is the key to both playing fast as well as having full control of your pick. Make sure that you are always using your wrist and not your elbow.
