I'm still waiting to hear the results of comparing the speed of your different layouts...
believe it or not, i haven't had time to revisit this until now. it's been a crazy holiday season...
anyway, i re-built my main layout from scratch, adding one element at a time to see if one of them caused the lag, or if it was the amount of elements that made it lag. my new layout was immediate with all the same elements!
so i realized it must be some specific setting and guess what it was -
Song Numbers in the Song list. having the Layout generate song numbers AND starting each "set" - which in this case was different tags - made it pause for the 15+ seconds.
again, the previous versions did not have this pause, so something changed. but i can easily turn this off for a Smart List situation, compared to a Set List.
so i guess this is solved for now

It will be the options in the song list that mainly determines the load time.
i see that you said this early on, but i forgot exactly what options were where to experiment back then.