It seems counter-intuitive, but when learning a riff you’ll find yourself racing through it (with a lot of buzzing and fumbling!) as you’re wanting to get the part played. It’s actually just as hard to play very slowly! Try slowing down your riff to an almost-boring BPM. You’ll find that you build up a much better understanding of where each note is and will help with ensuring each note ring out as well as possible. This also helps establish your dynamics throughout the riff and drills the visual identification of each note.
This goes without saying, but if you really want to play fast you should be practising as much as possible. A lot of guitar playing is all based on muscle memory and the best way to achieve this is by consistent practice. Set yourself a goal for 15 minutes every day (or every other day) to run through some hand exercises, playing exercises and practice with a metronome. A few weeks of this will level up a player of any skill!