Hello and welcome to the forum,
Each learner will have his own timing as well. Why do you not have something that pauses each slide and let the learner decide when to continue= a button or a click box? That is the approach I'm using mostly; even uncheck Calculate Caption Timing all the time. But of course it is your choice. I'm not aware of any setting you could change to increase all the timeline durations of all text captions with a certain percentage, sorry. That was your question, and I do not have the answer you want.
Most of my audience finds it annoying to have to click a Continue or Next button, and in my opinion it interferes with learning. You're right that each learner will have his or her own timing as well, but when I've published lessons with calculated timing, a high percentage complain that it goes too fast -- and when I've manually slowed them down by 10 to 20 percent, no one has complained that it's too fast or too slow. So I think there's a sweet spot, at least for my audience, and Captivate missed it on the short side. Thanks for your reply!
You can slow down or speed up playback by manipulating the frame rate of the movie, but this will also affect timing of voiceover if present.
Currently you can set captions with text to have Calculated Timing, but no further adjustment on the rate that this calculation applies. It's more of a ball-park figure so that you don't have so much to do. You're expected to drag the duration out to what you want. I would suggest that you log a feature request with Adobe for Cp6 that allows adjustment of the "calculation rate" for timed captions. A slider control might be good.