You are conflating two things that have nothing to do with each other. Computers have had mice since the 80s.

Still motion-to-photon latency is really lower on my DDR3 PCs, would be cool to know why.

No it isn't, your computer is doing tons of stuff and the cursor on windows is a hardware feature of the graphics card.

Should I even ask why you think memory bandwidth is the cause of mouse latency?

Dan Luu actually measured latency of older computers (terminal, input latency), and compared it to modern computers. It shows older computers (and I mean previous century-wise old) have lower input latency. This is much more interesting than 'feelings', especially when discussing with other people.