I've read on here numerous times if you want to pluck really fast, start plucking really slow, make sure your technique is 100% solid and then increase the tempo... I do not agree with this concept at all.
I seriously tried to do this for years, and for most of that time, I was a slower plucker than most. I have never kept the exact same movement nuances that I tried to maintain at slower tempos at blazing fast tempos. In fact, if I really did try to, I could not keep up that pace, and would have to slow down.
I never allowed myself to go really fast, because I kept noticing my fingers weren't as close to the strings as I decided they had to be at all times, my attack got more powerful than I wanted to, and all the other crap I told myself.
In all honesty, no one is going to maintain that stuff while playing at blazing fast speeds.
Also, you may make up some mental barriers, when you know you lose your ideal (slow) technique at say 16ths at 90 BPM. Next time a passage comes up and you know it exceeds that tempo, you will never be able to perform it well, because you'll be stuck with the knowledge that you can't do it, or the knowledge that your technique is "sloppy".
News flash, everyone's plucking technique gets sloppy at fast passages, but they still maintain the speed.
My advice for plucking fast is to not pay attention to what BPM your metronome is set at, just set the metronome to a fast speed and accomplish it, set it faster and accomplish it. Don't analyze every stupid movement your fingers, and knuckles make, just play at that speed and get quicker.
How to learn songs while playing them initially at a slower tempo? Don't just set the metronome to a slower setting. Download a program like Audacity for free, and slow the recording down to roughly 30%, nail it, and then only slow it down by 20%, then 10% then play along at full tempo. You'll have no idea what your speed limit is, you'll only know that you can play that really fast song.
The end. Start the arguing if you wish. I used to follow the strict rules you are arguing for and it messed up my development for years. Beginners, please do yourself a favour and don't listen to them.