If you were somehow able to solve each DROD room in 15 minutes (believe me, you won't) it would take you 1105 hours and 30 minutes to get through the whole thing.
There are rooms that will take you more than 15 minutes to solve, but I'll bet there are far more rooms that will only take a reasonably decent DRODer 1 minute (or less) to solve. Is 15 minutes a reasonable average? Maybe, maybe not, but the point about exponential growth of the number of holds is well taken.
Now, what order to attack holds? I would suggest (and this is from personal preference for harder holds) that you start with midle-difficulty holds (say 5 difficulty). If the first couple are pretty easy, ignore the others at that difficulty and move up. Continue until you find the difficulty level that suits you best. Of course, as you play more holds, you'll inevitably get better, so you'll need to move up some more. Once you get comfortable with the harder level holds, I'd switch over to rating.
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