I think it has less to do with the amount of time spent and more to do with the materials being consumed in the process. For example if you do Two Standards Synths for a toal of 900 exp in the same time it takes you to do one Careful Synth for a total of 900 exp you are STILL consuming TWICE as many materials, which is going to cost you time one way or another in the form of materials or money spent.

Now, I'm going to reiterate my above point. Why bother attempting to Careful Synth a recipe, taking twice as much time as you would performing a Standard Synth on the same recipe, for the exp bonus WHEN you could just move on to a recipe 5 lvls higher and spam standard (whilst using cross craft abilities) to obtain 900exp per synth in the same amount of time it takes you to standard synth a recipe that yields 450exp?