Good question. Keep in mind that the GMAT™ is a Computer-Adaptive Test (CAT), so the exam experience of one person may differ from that of another even if they were to land on a similar Quant score. Although to my knowledge, the algorithm of the test has not been released, I have read that it adjusts the level of difficulty based on responses to every block of two or so questions of a certain difficulty. I have also read that, despite what GMAC® says in the Official Guide, the first ten questions really do carry more weight than questions from either the middle portion (questions 11-20) or the end portion (questions 21-31) of the test. By the time you might reach question 5, then, you could already be facing a Hard question, especially if you have dealt with the first four questions accurately. Someone else might face a couple of Hard questions in the first set of 10 questions, miss only the Hard ones, and see-saw throughout the test between Medium and Hard questions. Remember, it is not how many questions you miss that determines your Quant score, but what difficulty the level of questions you miss may be--as well as that of those you answer correctly--over the course of 31 questions.
To see "devilishly" Hard questions, it would be necessary to hit a streak of several Hard questions in a row. Only GMAC® knows the exact percentage of test-takers who have answered any given question correctly, so I imagine that a block of Hard questions (say, two questions in the range of 20-30% accurate responses) might eventually bridge into two fiendishly Hard questions (say, 5-15% accurate responses) after correct responses are given to everything in between. I will say that in my Q50 experience, I missed one question in the first 10, and my streaks of correct responses varied between 8 (first portion); 6 (middle portion); and just 3 (end portion). That would seem to attest to the first 10 questions setting the tone for the rest.
In terms of improving your timing, it obviously helps to get your hands on a good content guide and to apply your techniques to official questions. (I do not advise people to work from unofficial sources, since the quality of the questions may vary.) If you enjoy self-study, then you must be diligent about tracking which problems you tend to miss or spend more time completing (e.g., PS vs. DS, or, concerning content, combinatorics or advanced geometry). When you have practiced enough questions, patterns will start to emerge, and you can pinpoint weaknesses to improve upon. If you have done all of this and still find yourself hitting a Quant wall, then it might be time to seek out either a study buddy who is way better at some of your weaker areas or a qualified tutor who can discuss test-specific strategies and source material to address any deficiencies.
If you have further questions, feel free to post them. Good luck with your studies.