There are limits. USB keyboards are just always going to be slower than PS/2 keyboards. You can close the gap, but USB is just fundamentally adding wrapping layers that introduce latency over the direct polling PS/2 used. And there's a lot of this at the hardware level, where standardized connections with complex protocols replace weird custom connectors. USB is most definitely easier to plug something in to than PS/2 ports, where you had different ports for mouse and keyboard and they weren't interchangeable. (Ok, some very late motherboards supported using either in a single port, but it wasn't the way most PCs worked.)

You can do a lot at the high levels, but everything needs to be reworked from the hardware level up to match the latency of older systems.