This may sound completely insane and the exact opposite of what you have been told by every music instructor you have ever studied with. If you are looking to improve your speed. Rather than starting off really slow and gradually increasing your speed, there is another approach.
Most people will tell you to start of very slow and make sure that you are able to articulate every note. Then--and only then--increase the speed on your metronome. This is true to a point, especially if you are just getting started. However, this will only yield incremental results.
I once took a speed reading course. The first exercise of the course was to take a baseline reading; read as fast as you can for a period of time. After the bell I recorded the number of lines I read. Next, they had us set the metronome at a very high speed. The drill was to scan a line of text with each beat of the metronome. They didn't want you to read each word, only to scan the words at this impossible pace. A funny thing happened: I started to understand and comprehend the words that flew by. At first it was in fits and starts, but my mind began adjusting to this new speed. After 10 minutes of conditioning they had me reread the passage I read earlier to find that I doubled my reading speed.
The same works for guitar (once you have the fundamental skills down of course). If you set the metronome to something that you have to really struggle to keep up with and try to keep pace--even if it is sloppy--you are training your brain and hands to work a little differently. You quickly adapt to the new conditions. After attempting to keep pace at a higher speed you should notice that it is easier to play at the speeds you were just struggling with.Â
With the old approach, you are trying to do the same things you are doing currently, only a little faster. The problem with this approach is that you most likely have some bad habits, issues with economy of motion and efficiency. By barely bumping up the dial and repeating these same motions, you are only perfecting these habits. There are physical limits to this as well.Â
By speeding this up significantly your training wheels will fall off, your crutches will be painfully obvious and you will abandon them in order to keep pace.
This might not work for everyone. But if you have hit a wall, it is well worth a try. I don't always play fast, but when I do I prefer to play it cleanly and with feeling.