Went through DUB pre-clearance yesterday on the CO flight to EWR. It took an hour from entering the terminal to get to the departure gate having to go through check-in, security clearance and finally INS pre-clearance without stopping anywhere else in the Terminal. That was on a day when DUB was not very busy! The airlines insist you clearance INS at the latest, an hour before the flight, otherwise they can refuse to board you. Not sure they would but they can.
Overall, I have found that DUB preclearance is very crowded and does not save anytime, unless you did not check bags, as you have to wait at your first destination in the USA to collect your checked baggage to clear customs. Also, I have found numerous times in DUB over the years that many of the INS agents are not at all friendly which is surprising when you compare it somewhere like EWR or ATL where I actually find them more pleasant. Not sure why.
Hope that this helps.