Sorry, that is ridiculous. If there are too many tasks while taxying you stop them or set the parking brake. All of the crew pays attention to the here and now. Taxying in an unknown area with minimal lighting (if it was well known to the crew or well lit.....) all heads are up. No exceptions.
I'm sorry are you a private pilot?
Airlines usually provide the performance numbers through ACARS or they are computed on laptops/tablets prior to pushback. The only task would be getting a route clearance (if not obtained prior to push) and the checklists.
I'm sorry but Nigel really did bad for the rest of perfection on this one. Pull the FOs head out of .. and have him look out the window.
Distraction from the primary task of taxying safely.
These days airline crews are given far, far too many tasks to do while taxying out to the runway.
It's become a macho thing: "Yes we can handle any task you give us while navigating to the end of the runway".
But be honest. Who hasn't looked up while taxying - and got a bit of a shock? Yes?