There are a few different ways to strum fast on a mandolin. The most important thing is to use your fretting hand to help keep the rhythm steady while you strum with your picking hand. One way to do this is to use a "rolling" motion with your fretting hand, where you roll your fingers across the strings as you strum. This can take some practice to get used to, but it can really help keep things steady. Another way to strum fast is to use a "bouncing" motion with your picking hand. This means that you quickly bounce your pick off of the strings after each stroke. This can be tricky to get the hang of at first, but once you get the timing down it can help you strum faster. Finally, make sure to practice regularly so that you can build up your speed and stamina. The more you play, the better you'll get at strumming quickly. Just keep at it and soon you'll be playing fast mandolin licks like a pro!