On my Canon 70D there's a considerable lag during the change of aperture when pressing the depth of field preview button. This lag also occurs during video recording, which causes the video to halt shortly when the aperture changes.
This happens on the Sigma 18-35 f/1.8 lens but seemingly not on the Canon 40/2.8 STM.
Do different lenses have different ways of communicating aperture changes, or is it more likely that the latter lens changes the aperture rapidly enough not to cause any lag?
(Although this is not really an issue unless your shooting videos, I hope that the question is general enough to be on topic here.)
I checked again, and the Canon 40/2.8 actually does behave the same in live view when pressing the DOF preview button, i.e. causes lag. But when shooting videos, the aperture changes smoothly, which is not the case for the sigma lens mentioned above and Canon 50/1.8. The latter two do produce lag during filming when the fast aperture shifting occurs.
I've seen that the Canon 40/2.8 is marketed as a lens for movie shooting because of the quieter focusing due to the step motor, but in my opinion the smoother aperture change is MUCH more important. The Sigma lens is more or less useless because of this, which is a pity considering its overall quality and well-suited zoom range for recording videos.
The same lag also occurs with Canon EF 50/1.8 and Canon EF-S 55-250/4-5.6. Very annoying!