I had a teacher in the past who used this analogy: if you're driving on the freeway and the speed limit is 70, would you rather be driving a car whose speedometer goes up to 90, or a car whose speedometer goes up to 140? Probably the latter, because the first car is going to be really straining to keep up with 70 miles an hour. And what if you have to speed up, to pass or something? The car with the higher top speed will do that with ease, but the car with the lower top speed won't.
His point was when you are trying to play a fast passage with fluidity, you want to be the car with a top speed of 140. You want to be confident that you could play the passage even faster than the required tempo, so when you play at the right tempo it feels effortless.
For this reason, when you practice fast you should make a conscious effort to aim for fluidity. Slow practice is where you hone your technique, figure out your fingering and picking (I don't play guitar, is that something y'all have to think about?), etc. If you practice fast, you probably won't be able to be perfect every time, but the point is just to get comfortable and play with ease.