That's a myth that should die out. It used to be true, decades ago, but not anymore.

The PS/2 protocol interrupts the CPU and sends a packet. USB has the CPU poll the connection and then gets the packet. However, the polling and clock rate of USB is so high that it can hit it several times before the PS/2 is done transmitting a single packet.

NKRO is also no longer an issue in newer USB versions. You have to get a more expensive keyboard to make it work--cost of all the diodes adds up--but that was just as true of PS/2.

Here's a Ben Eater video that goes over the details with an oscilloscope: https://www.youtube.com/watch?v=wdgULBpRoXk