about converting ZX-Spectrum games to MSX as they were in the late 80’s or early 90’s

These are called "lazy Spectrum ports". Of course it wasn't really about laziness. In many countries, the MSX share was considered so small that companies weren't willing to invest much on these conversions. I remember reading that a coder was given just three days to make such a port

So, it wouldn't be so hard to exceed that quality, at least. But of course, when the game sources are not available, there will be some challenge disassembling and researching the Spectrum games in question.

In the past, I have done one of such a conversion. But it was my own game, so I had sources of it and knew the game logic well. And it was a simple game. And I still used more than three days doing that

I do not agree woth you. To do sw sprites on zx you need some small buffer to avoid flickering when updating the screen. This involves a copy of contiuous byte interlaced from random access. In this situation the loss of speed is not so remarkable.
If this was as you described there will. Be no zx - msx ports because nearly impossible.

Consider that a vram vpoke is nearly slow as 8x ld r,(HL)

That weird layout is not a problem, the conversion to MSX screen 2 format is pretty straightforward. The slower access to the screen memory is a bit more of a challenge. Indeed, many Spectrum ports are slower on the MSX than the original on the Speccy. However, the screen access speed is not a show-stopper, when coded properly. And many Speccy games use off-screen buffers already, so very close to the original frame rates are possible, if digging the mechanism deep enough.

I think there are bigger challenges in disassembling and analyzing those games. Many of them have strange interrupt routines, strange usage of the Spectrum ROM, and weird mechanisms for making piracy harder. But nothing impossible of course, I'm sure they all are hackable, it's just a question of how long it takes.

Yeah. The screen 2 mode has 8 times more bytes in the colour attribute table, than the Spectrum. We'd need some way to update the changed "tiles" only. That should make it fast enough.

About sprites, one idea would be to use the Spectrum-style software sprites almost as is, and just use hardware sprites to add some colour to the main character and other important sprites.

And, by using MSX2 9938 VDP, even more could be achieved.

Edit: One more problem came to my mind: in the Spectrum, there are no system variables in the high memory area. The MSX keeps important variables and buffers in $F380 - $FFFF, with Disk Basic even lower. Almost any Speccy game would just overwrite them. So the MSX BIOS should be deactivated, and replaced with a custom "bios" and ISR that don't care about that memory area. Still, the game code would need to be hacked not to use the address $FFFF, because it's the extended slot selector.

IMHO, the best option would be to do proper ports of the good spectrum games to MSX2, pretty similar to the Alien-8, Knight Lore and Batman remakes for the MSX2. Ok, I know those were converted from the MSX1 versions, but the process would be pretty much the same.

Even action games are feasible, like The Last Mission and Goody for the MSX2.