Like if you have to concatenate string to make labels (and then parse and measure and format and re-flow text), you end up generating a lot of garbage and memory management and pointless recalculation in the main loop.

Yes, for sure there is some overhead in this. I checked in Tracy now and with a lot of gui open, using many custom labels etc, it takes on the order of 150-200 us to do that part of the gui. Issuing the GL draw list commands takes some time too, but not too much. I think the expectation is that it is expensive for real, but it turns out that ImGui is actually quite cpu-time-cheap. And a "modern" electron based app gui has a whole different set of overheads, so it not like all alternatives are overhead-free wonderlands..