alt Hacker NewsRAM latency doesn't affect mouse response in any perceptible way. The fastest gaming mice I know of run at 8000Hz, so that's 125000ns between samples, much bigger than any CAS latency. And most mice run substantially slower.
Maybe your old PC used lower-latency GUI software, e.g. uncomposited Xorg instead of Wayland.
I only felt it on Windows, maybe tht is due to the special USB mouse drivers Microsoft made? Still motion-to-photon latency is really lower on my DDR3 PCs, would be cool to know why.