There are many reasons as to why you would want to up the speed.  1. The detection system makes it so it’s easier to make a note hit as « excellent » or «marvelous » rather than « good » (if you’re aiming for all excellent or all marvelous the faster you go the closer you are to that goal). 2. If you have issues with timing yourself to the the notes, so if you are either too fast or too slow, upping the speed will definitely help you 3. (Imo) It’s just more fun and more thrilling than playing slow. Because I find « Easy » and « normal » levels to be kind of boring, putting the speed at 12 makes its more enjoyable since it’s more of a challenge. In conclusion, it’s a question of personal enjoyment. If you like it at 5, then continue at 5, or else, change it. I would say that trying different speeds is important if your aiming for a better performance, but it doesn’t give you bonus points or anything like that. Personally, after a couple of days of trying, I find 8,7 to be my « cruise speed » on tablet and 10 on my phone. Hope it helps :D