The PS/2 port runs off the legacy keyboard controller, which is an 8-MHz, 8-bit ISA general purpose I/O controller with some firmware in it. (Yes it still is, even though there isn't any physically visible ISA bus in today's PCs.)
In other words, it's the oldest and slowest piece of hardware still in there, dating back to 1977 or so. Running your mouse on USB will impose much less bus traffic.