Forgive me if I'm missing the point here (and I'm just a dabbling programmer, not at the level of you guys), but it seem there are a lot of vertical scrollers on the CV, like Bosconian and Rally-X.
I'm seeing a lot of talk about not being able to scroll vertically quick enough, but those two games do a decent job. I'm sure there are more. What did they do differently? Is it just a matter of having too many objects to scroll? Is it because (particularly with those two games) only 2/3rds of the screen actually move? Would a narrowed playfield help?
It's all about the level of detail in the graphics. I can't speak much about Bosconian, but I suspect it's the same story as Rally-X, where the background graphics are so simple that it's possible to store all the pre-shifted tile patterns in VRAM, and then it's only a matter of updating the character tile table (or as we usually call it, the "name table") in VRAM to create the illusion of scrolling.
With Commando, the graphics are so detailed and varied that it's impossible to store all the tiles pre-shifted in VRAM. The I/O link between ROM/RAM and VRAM is very slow on the ColecoVision, so it creates a bottleneck where there's not enough time to update large portions of the VRAM between TV screen refresh cycles. If you update VRAM only partially between refresh cycles, you see lots of graphic glitches on the screen, which is not desirable.
So that's why we're lamenting that Commando is hard to do on the ColecoVision. I've given it some thought this past week, and I think it's possible to do, but I don't have time to expose my ideas on the subject right now.