I'm using a RISC-V board, and I find that with one distro, X windows (based on Wayland) is acceptably fast, although I can't use it because "Plasma" frequently crashes.

When I switch to another distro (Armbian for RISC-V) that has Xorg, the framebuffer memory is so slow, it is literally a fraction of the speed of how Xfree86 was on a 80486DX in the 1990's. Does anyone know what could artificially slow down the framebuffer memory? I'm assuming at this point it is shared main memory. When I tried to use Wayland with this distro, some very buggy behavior resulted so I am stuck using Xorg.

Both distros are using the same kernel. There is no video driver as this point, it's just /dev/fb0.

what I can imagine is the amount of available ram, or something similar (low on resources?). But without details extremely hard to say anything. From the other hand you may try to rebuild Wayland for your system, probably that will be more stable.

A lack of RAM can't be it, because the board has 8GB and something like 6.5GB is free.

Also the way in which video RAM is updated is suspicious. When Xorg draws there is a diagonal tearing that occurs.
For example, when a rectangular area is to be filled with a color, this filling occurs out of order, symmetrically along a diagonal. It looks horrible but it is also super-slow.

As for Wayland, I did rebuild it and it was even buggier than Xorg but for another reason: When I type a character in xterm, it is repeated maybe 10 times. I'm surprised such bugs are even possible.

My guess is that video RAM is somehow being managed by the MMU when it shouldn't actually be, but right now I'm unsure how to prove it or fix it.