#1
I played guitar for several years in the eighties and nineties and mainly played rhythm guitar, not so much lead. I took several years off of playing too. Now I'm back into playing and am practicing at least an hour and a half a day sometimes more. My rhythm playing has gotten back to a pretty good pace already, but I am also working on lead now.

I do use a metronome, and I do tons of excercies from Troy Stetina, John Petrucci, Paul Gilbert, Guitar World, etc.......

The problem that I have is that I can play much more efficient and faster when my left hand is fingering in the ascending direction. (example=1234, 123,13,12,14,134, etc...) , but when I go backwards or descending I play a little sloppier and not as fast. (ex=4321,431,etc....)

I do play slow and then speed up.

On chromatics I play at 120bpm on 16th notes clean back and forth. Forward on it's own I can play 150 bpm on 16th notes.

Should I just practice mainly on descending since it is much weaker.
Thanks in advance.
#2
If you have an obvious weakness, target it and correct it. What's funny is that you may practice this so much that in a little while, it is actually your strongest technique. To practice, start slow (always) and concentrate of being clean. Only when you can play PERFECTLY at a speed should you try it faster.
#3
bgc is right, i'm currently working on my ascending playing cuz its not as good as my descending playing (so between me and the TC we have a decent guitarist) but yeah, he says start slow, i would say just start at a speed you consider comfortable that you can play cleanly at.