I actually bought the Mr. Monk's Guide to Exhaustion Codex because I find grouping enemies together and taking them all out with charge attacks to be a very good strategy, but you run out of stamina very fast.
However, I simply cannot see any difference whatsoever with it equipped. Charge attacks take the same amount of stamina. I still can't do charge attacks with minimum stamina. My stamina still regenerates at the same rate. My stamina still seems to deplete at the same rate. I really don't get it.
Something I noticed while testing was that your stamina starts regenerating as soon as you cancel out of your current animation. If you press the attack button and let the full animation go off your stamina doesn't start to fill back up until the animation is finished, unless you cancel it by running forward. Which means you can start your stamina regenerating much faster by moving out of animations early.
(http://recordit.co/cBW4md69F8)
The other Codex I bought was Carpe Diem which is EXTREMELY powerful. I had enough gems to buy the max amount of blessings (I think 4? 500 > 1500 > 2500 > 3500) before level 4. Since the God's Blessings are really the only thing worth spending gems on I suggest equipping it for the first few levels to max out blessings and then switching to a more useful Codex. It seems most people say the Vampirism one is the best, which is probably true; but it would feel cheap to me and it's really obvious what it does.
It's very frustrating that the codices are given vague tooltips given the amount of effort required to unlock them, especially when you buy one (Like Mr. Monk's) and still can't figure out what it does despite testing. I'd like to try to map out what they all do, but I think I'll lose interest pretty fast.
I've not encountered the problem you're describing, but you should try to google for it. If that fails, use Can I Run It to make sure that your system can actually run the game. If that succeeds, the fuck with your video settings in the main menu. If all else fails, try Harebrained Schemes support!