It's a basic principle advocated by everybody - you have to be able to play slow before you can play fast. You have to be accurate and in time at slow tempos before there is any point in trying to crank it up.
Speed is a by-product of good technique. I spent my teenage years doing exercises to get faster, but I've seen greater improvement overall since I hit my 20s and realised speed of itself was not a goal worth pursuing.
I still do speed drills to push myself, but only now and again instead of constantly. I'm now not very interested in the mechanical aspect of guitar playing anymore, but more on the theory end, modes of other scale types etc etc.
I've been playing for 22 years now, and I still practice 2-3 hours a day 99% of days. I'll probably never stop learning.
Only other observation on slow practice would be to practice at a speed you find difficult. There are licks I can play at 40bpm and 200bpm, but for whatever reason I screw them up at 127bpm - so that's what I'll practice. Always practice the stuff you can't do - sounds obvious but so many people keep going over the stuff they can do, as it's more immediately rewarding...