Yep, every music teacher says it, but it’s so tempting to play the easy bits full-speed ahead only to stumble on that fiddly phrase. Practicing slowly can be so effective though, so it’s worth persevering. Muscle memory takes time to develop.
A conductor likened playing a tricky passage to regularly parking a car tight spot on the driveway. The first few times you have approach it slowly, work out the moves carefully so as not to collide with anything. But after a week or so it becomes automatic and much quicker.
It’s the same with playing an instrument, your fingers have to get familiar with the set moves required to play a certain group of notes.
Using a metronome can be very helpful. Set a slow speed, practice that awkward passage every day and increment the metronome speed setting a little bit each day. Before you know it, you’ll be reeling off that complex run of semi-quavers at break-neck speed!