I was just reading up about this a couple days ago. The PS/2 comeback for gaming keyboards stems from the fact that the PS/2 connector is interrupt-driven from the device, whereas USB is polled from the host. So in theory, PS/2 latency should be lower. But in practice there are numerous other factors that tend to outweigh any potential benefit: key debouncing logic time generally overwhelms the total latency, the low ~10kbps throughput limits the effective minimum latency of PS/2 bus messaging, and the fact that even if the bus is interrupt-based, keyboards use polling internally to scan the state of the key matrix. It turns out that the implementation details of the how a keyboard scans the matrix is generally a bigger factor in keyboard latency than which bus is used.