I put the faster than the devs one to stop myself from lowering the overall time. I realized while developing that I was a terrible measure of difficulty but if I turned on that mode I felt challenged so could leave the regular mode alone. That mode really means you have memorized the puzzles so can immediately place pieces.
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My kids played and got all the achievements. One of them discovered that if you had Dev Time selected before starting a new game, and then paused the game, you could select Ignore Timer and visa versa, and have them both show as being active. The Ignore Timer seemed to take priority, regardless of which order you did it in.