Back when I was playing AD&D 1st edition on a nearly continual basis, there seemed to be two kinds of rules.
The first set of rules were the ones that got used all the time at the table during play. Combat, spellcasting, movement, climbing walls; that sort of thing. That was the sort of stuff that we either had memorized, could flip to the proper table in the DMG, PH, or UA by reflex, or just hand-waved.
However, there was a whole other set of rules that by their nature didn’t really come up during the actual game, but had to do with things that happened during down-time. This is when we poured over our character sheets making sure we had enough sacks for our coins, and our backpacks could hold all the iron spikes we carried. When we calculated how many pages our spellbooks had to be to hold all our spells (and whether we needed a traveling spellbook or a regular one). When we figured out height and weight and all that stuff. When we calculated the cost of building manors and fortifications, and hired mercenaries (were hobilars the most efficient use of our money?). Hired criers to advertise for henchmen.
All that down-time activity was incredibly fiddly, and really did require us to stop and read through rules that we didn’t use all the time, and carefully work out costs and such in the days before Excel. But it didn’t matter, because that was the sort of thing we did on our own, in between games, and if there weren’t miniatures that needed painting the time might as well be profitably spent figuring out the total carrying capacity of our hirelings (thanks to the carrying capacity rules that for some reason were only found in the instructions on the Permanent Character Record.
And when we were done, the DM didn’t even bother to check the work, because we all trusted each other, and we got on with the game. But those fiddly in-between bits were like a solo game unto themselves, and there are times I miss having the free time to while away the hours figuring out those minutiae.