In F, BA are offering personalised PJs (size) and amenity kit (gender orientation of the contents). That precludes loading them before takeoff and getting it right every time because some passengers will always be assigned seats after the aircraft is cleaned and catered and it is worse to deliver the wrong thing than to deliver it later. BA are also trying to offer a personal service where the staff give you something rather than having you find it - and it's hard to disagree that the F service on BA is more personalised and attentive than the AA F service.
In J neither airline is offering a personalised amenity and PJ service so they could place the amenity kit and menu at the seat beforehand - and AA does so.
In F, and even in J, many passengers are looking forward to a relaxed, restaurant-style service (and I don't mean Wagamama!). Some may want a faster service, and the way most airlines that please both types of passenger do so is to have an "express menu" option where items are brought out together or some courses are omitted. "Why does BA service take longer" is not least because "BA consider most of their passengers want, or will accept, it that way."
That said, some BA services are really very slow these days and that's not good. Ideally, like in any good restaurant, the BA meal service should have: prompt taking of orders and prompt delivery of drinks; starters a short while later; main courses when the starters have been finished for a few minutes; dessert offer a little while after the main course has been finished; post-prandial drinks and dessert delivered at about the same time. If the guest wants to slow things down, they can delay indicating that they have finished (cutlery position!).