There is no speed advantage. Your on the right lines with the lookup table but your logic for how we get there seems a little off. First of all, whether we have to request one, five or one hundred numbers from the RNG is irrelevant - it can spit out hundreds of thousands of them every second without issue, the only bottleneck is the data connection - the CPU in your computer can create random numbers faster than your internet can send them (or, for that matter, your hard disk can store them). Next I'm not sure theres any multiplication involved in either case? Maybe I misunderstand somewhere? Though even if they were, again these calculations are so simplistic for a computer to perform, it would take millions of users to saturate the capacity.
When a game is being developed and the maths model is created, an essential element of that process is to determine the games RTP, to do this necessitates recording every possible outcome of the reels so why not store that data in a database for future use? Infact, it's extremely likely that the slot designer says okay we want 96pc tRTP, please give me reel strips that match this value, and their engine goes off and creates these strips.
Also consider the fact most games have adjustable numbers of lines and show you which lines you have scored a win on after each spin, this data also needs to be recorded, every line win for every possible combination of spins - and would be required whether we start with 5x reel stop positions, or a win value as your suggesting.. the difference if we were to fetch a win value is we then have to make another request to choose one of the possible outcomes that match that win ... a second, unnecessary stage to the procedure.
Of course I can't guarantee that the slot designers make the games this way but I can say what makes sense from a programming perspective, and if there is no financial advantage for the designer to deviate from the most efficient procedure its usually fair to say that they will follow that route, especially if it is the simplest and most efficient anyway.
So, we fetch our five stop positions, in our imaginary gam we get back from our RNG stops 5, 12, 17, 30, 2, in our database there is a unique row with those five stop positions, and linked to that row is a second table with the outcomes for each line visible with those stops.. no calculations are required, it was all worked out at the time the designer fixed the RTP of the game.
With regards the rolling reels bonus round, the reel strips are altered for the bonus round as your probably aware, but the layout of those reels will be generated by the computer in order to meet a specific RTP, how easy would it be to get a 'sweet spot' 3 of a kind? Pretty much the same as in the main game, but in order to give slightly larger wins we have several runs of the same symbols on the first three reels... I maybe don't understand your point completely because I don't really see how this bonus round would confirm your idea that developers work backwards from a win number to reel positions - its just massively simpler to do it the other way around, so unless anybody can think of a financial advantage to doing things that way I can't buy into the theory