I am a new FO on the 73 and have had mixed experiences. Usually we do our perf during or right after push before we tax but about 20% of the time the perf comes later and we have to do it during taxi. Usually if I'm heads down its because I'm looking at the taxi diagram trying to make heads or tails of the taxi instructions. I had one experience coming out of SLC where we got late perf, had to switch runways, requested new perf, and the new numbers created the need for a bleeds off takeoff. There were so many things we had to change or brief that I spoke up and asked the Captain if we could pull over and sort it out. That sort of broke the chain, we parked for 3 minutes on the deice pad with ground's permission and got straightened out.

Bottom line IMO, is if the FO is feeling the pinch, he has to speak up and let the other guy know. No awards for a 3 minute earlier takeoff but they'll come after you for the mistake.