Yeah, from what I've observed it is only 3 or 4, I haven't seen any 5 frame updates (and I hope I don't
), it's just finding any rhyme or reason about it, screen scrolling itself definitely tends to set it to 4 for some levels, but what else affects is still a mystery
1507-1511 is the first time I see a 4-cycle rather than a 3 too, so this is going to be what I focus on
The thing that happens on this frame is the 50th and last lemming out's sprite is drawn (or the 49th still shown on screen)
A picture of frame 1511 is shown, with sprites (excluding the cursor) counted
I'm going to try and reduce the RR part way through the level and see if this causes less delays
EDIT: Yes this has stopped the 1507-1511 4-cycle, 20% saved is now at frame 1949 saving 43 frames (with no screen scrolls at the end), in fact doing this has removed all 4-cycles in the level, so it's definitely something to do with either sprites or keeping track of lemmings. (Scrolling still causes 4-cycles at the end though)
EDIT2: Decreasing the release rate on Fun 3 (Blockers) didn't save any frames despite having the same number of lemmings, it has less object sprites so I'm going to assume that's why.
EDIT3: There are some 4-cycles in Fun 1 when a lot of digging is going on, not sure what's causing these or if there's anything that can be done about them. I have a feeling it's the particles from the digging, scrolling the screen left at the end saved 1 frame. Some more frames may be saved if I scroll the screen as left as possible at all times. (Due to a lack of objects or lemmings or sprites screen-scrolling doesn't seem to delay this level)
EDIT3.1: No more frames were saved that way.
I attached my current memory watch list to this post too which makes things a lot easier to see (it has each lemmings x-position etc)