Fast left hand requires loads of practice. Intonation is harder than faster even at slow speeds. Practice with the Bow. Learn all of Simandl and every other book like it. Learn the Dragonetti. Practice what you can of those with Pizz as well but Bow first. The fingers can't play what the left hand can't.
Get a Teacher that is a professional Orchestra player. It will make a world of difference. Been there, done that.
I lost some speed but gained accuracy. I made more money as a pro player in NY with the accuracy. The speed didn't pay so well!
I actually prefer the feel and tone of Ray Brown to the sound and speed of NHOP. He is great but I would rather listen to Ray if I had to choose between the two. Don't get sidetracked by speed. Find some clips of Ray Brown and study him musically before you start the race. Walk, don't run!