That depends. How many notes does the riff have? What is the fingering? Hard or easy fingering?
Here are two pieces that are standard showcase/practice pieces for guitarists. They are both really hard to get right for different reasons.
Recuerdos is 1/32 notes at a moderate tempo, and still a beast.
Very few people can get Jerry's break down right--and it is 1/8 notes at about 276...
So, think of it like this: anything with 16th notes at 130 is going to be hard not to play sloppy unless you SLOW IT WAY DOWN TO PRACTICE.