On the original ZX Spectrum, you could measure the write bandwidth visually, because on startup it would write the value 2 into each byte in memory (which included the graphics RAM). It would then re-read and decrease the value of each byte twice, to check for any faulty memory.
You could see these patterns on-screen as the reads and writes took place (I think it took about a couple of seconds to do this to 48k of RAM)
You could change the stack pointer to the top of the area of memory you wanted to fill and then use PUSH to fill at I think 11 clock cycles per two bytes. It was faster than unrolled LDI or LD (HL),A followed by INC HL. It would be filling memory in the wrong direction for a Rainbow processor but you could use it for repeating patterns. I think I did a checkerboard pattern that would shift every frame and it was pretty smooth.
You could see these patterns on-screen as the reads and writes took place (I think it took about a couple of seconds to do this to 48k of RAM)