Either the delay should be ignored, like with Mate's screenshot app, or the delay should happen and then allow a user to select the region, like with KDE's screenshot app. Having the delay happen after you've selected the region, which could take longer than the delay time, doesnt make sense.
I second that, it's very annoying and makes very little sense to delay the screenshot after the region has been selected... I know for sure that functionality was different some time ago, thus the screenshots were taken immediately after the region has been selected. Some people still may find it useful, therefore ideally there should be a checkbox to disable "Delay before capturing" when Selecting a region.
Andre has agreed on IRC that this can be implemented ( delay -> select region -> capture ) and in order to preserve the current behaviour we'd have a checkbox and CLI option to toggle between the two behaviours.
*** This bug has been marked as a duplicate of bug 15801 ***