IMHO to know where is the bottleneck we need to profile which commands the cpu send to the vdp.
In 8x8 block rendering i agree, the cpu may be the bottleneck, but in this game the vdp appear to do a single big copy operation. And this maybe the show stopper. Maybe one can avoid this by using double buffering but there is also the need to scroll.... and screen 7 is somewhat 54Kb of size.
Even if uses a big copy, if it waits for it to finish (as I imagine it does) instead compute the frame game logic at the same time, is something similar. Gain some time because big copy is faster for VDP than many small ones, but the CPU is idle in any case.
The problem in screen 7 really is just the time to complete the copy in the first place, due to the large amount of data. Can be mitigated (to some degree) by aggressively tiling if you scroll on the grid, however since this game scrolls in smaller steps it can not do that. You can also tell from the video because you can actually see the copy execute from top to bottom over several frames.
What is also sad is that this game has FM sound on the PC-88, and only PSG sound on the MSX2.
Indeed, as many titles, especially from Falcom, T&E Soft, Telenet and Wolf Team, but also others. Sometimes I like to dream how "Testament" would have sounded if it had FM support (and I want to re-arrange everything some day).
Falcom games with only PSG should be considered as a crime. Even on PSG sounds good because the composition is really nice, but a waste for all those JDK Band tracks. DS6 with FM: https://youtu.be/W28McQJlY6w
Well, in the case of "Sorcerian", they would have done better with not supporting FM at all. The MSX2 FM soundtrack of that game is a complete trainwreck. Probably a quickly done port from the PC-88 version, with obviously not too much checking before releasing. You're better off disabling any FM device before playing the MSX2 version.
Romancia and Dragonslayer IV sound pretty cool with their PSG-only soundtracks. But especially concerning DS4, we can only dream how it would have sounded with OPLL support (but I'm taking my chance behind the scenes in re-arranging that also).
(I do think the PSG renditions of the Ys games' music on MSX2 are very well done, by the way)
