Humans improve the quickest by pushing the hardest.
The correct way to practice tactics?

the advice i've been given by strong players has been conflicting. One 2000 said to do all 3 versions of those. The tactics that you can only spend 3 seconds on, the tactics you should spend under a minute on, and the combinations that may take you a few minutes to find
IM Dpruess says you should do only 15 minutes a day and if you dont get the tactic in under a minute or you get it wrong you stop trying to find it, look at the solution three times, And then visualize the solution in your minds eye 3 times

Soviet chess training methods and Dan Heisman among others disagree. They feel that training on easy tactics again and again until you nearly instantly recognize them gives the best benefit. Then you're able to combine them into the harder problems.
A rough analogy is multiplication tables. Anybody who's decent at math can derive the right answer for 8x7, but more complex 4 digit multiplication is only reasonable in limited time if you instantly know the answer to 8x7 without working it out.
On chesstempo.com, the site I use to practice tactics, I am rated about 1690(fairly close to my most accurate rating of 1600 fics standard). The problem is that I am having trouble deciding which way to practice tactics. I have recently moved to the hard set of tactics (there are 3 difficulties: easy, medium, and hard), but I am not sure if I should go back to medium. When I use hard difficulty I can correctly solve about half of the problems, sometimes more, but it takes me around 5 mins to solve each problem. When I use medium difficulty I can solve many more because it takes me about a minute and a half to solve each.
So my question is, which difficulty should I use? I have to challenge myself more on hard, but when using medium it might help my pattern recognition since I solve more (but easier) puzzles.