Timing:
Here's one article:
http://www.beatthegmat.com/mba/2009/12/ ... managementI think you would also benefit from a timing exercise: learning about how long one minute is without looking at a watch or stopwatch. If you don't have one already, buy yourself a stopwatch with lap timing capability. When you go to do a set of problems, start the stopwatch but turn it over so you can't see the time. Every time you think you're one minute into a new problem, push the lap button. When you're done with that problem, push the lap button again, then repeat the process for the next problem. When you're done with the set, see how good you were - and whether you tend to over or underestimate. Get yourself to the point where you're within 15 seconds either way on a regular basis (that is, you can generally predict between 45 sec and 1min 15 sec). Also check your timing for the entire question, of course.
Now, how do you use that when doing problems? If you're not on track by one minute, make an educated guess** and move on. (The general idea is that if you're not on track by the halfway mark, you're unlikely to figure out what's holding you back AND have time to do the whole problem in the 1 min you have left.)
** This also requires you to know HOW to make an educated guess depending upon the type of problem and the content being tested. So that's something else to add to your study: how to make educated guesses on different kinds of problems.
Here are a couple of articles on educated guessing:
http://www.beatthegmat.com/mba/2010/07/ ... s-on-quanthttp://www.beatthegmat.com/mba/2010/08/ ... -on-verbalFor CR, no, it's not required to change anything... not if you're already okay with CR. The general rule is: if something's already working for you, keep doing it. If you want to get better, however, you need to change somehow.
The point of the diagramming is to understand the purposes of the information given, and to organize the flow / understand the connections between information. There are many ways to do that - the T-diagram is just one way. So the overall point, yes, that needs to be done somehow if you want to be really good at CR. But you can adjust HOW you actually do that.
I know people who draw little pictures or timelines. I myself tend to just write the info in order and the use arrows to tell me what leads to what. And many people write a lot less than what's shown in the book. My notes are so short that, if I were to read them again a couple of days later without the argument, I'd have no idea what they were talking about. :)
Finally, you mentioned making some careless mistakes - this article might be of use:
http://www.manhattangmat.com/articles/error-log.cfmFocus particularly on the careless errors section of the article.