Depends.

If the pax has a genuine physical disability, then a wheelchair may well be faster than walking, depending on the gate. It can be a really long walk to immigrations if you get the wrong one, and then almost as far from there to domestic departures.

If, on the other hand, the wheelchair request is just a preferential treatment scam, then walking will be faster. Pax requiring assistance will deplane last, and I've seen over 40 wheelchairs waiting in the jetway upon arrival in JNB. I kid you not. Most pax will already be at or through immigrations by the time the wheelchair show gets on the road.

I've taken a number of BA longhaul flights (in F and J) in the company of a person needing a wheelchair, and in all instances it took (much) longer than it otherwise would have. The only theoretical advantage was priority boarding, which we had anyway.

Johan