AFAIK splitting a KB & Mouse onto different Host controllers will increase mouse accuracy and alleviate Host controller congestion. For an experiment take both your devices and run them on the same USB Host hub and then try them on separate Host controllers during gaming. For me this is noticeable even on two 1K-Hz devices. Some games might not exploit the "problem" but for me it's noticeable. It became more apparent with 2 8K-Hz devices (M & KB) This has been demonstrated via MouseTester in a troubleshooting thread.
You are confusing polling rate with bandwidth.
So your standard mouse takes 4 bytes to send all its data. That is wrapped up in a USB packet, which will make that about 8 bytes in size.
An old USB 1.1 connection from the beginning of time communicates at 1,500,000 bytes per second. Which means there is enough bandwidth on the 1990's USB channel to poll a mouse or keyboard at 187500 times per second.
The 1000 Hz ( or times per second ) polling rate is a limitation of the mouse or keyboard.
You could put over a 100 modern gaming mice on a USB port from 1999 with no lag.
Your just seeing a placebo effect.
So with 4 PCIe 4.0 lanes going to the X570 chip how is this any different than a HEDT with it's 128 lanes, I mean of course the lane count but,, What does the X570 platform lose with the X570 chip? IIRC isn't it the 12nm IMC chip?
Lose?
For the home user, nothing.
HEDT is for systems with
3 or more video cards
5 or more NVMe drives.
48 or more USB 3.0 ports
384 or more USB 2.0 ports
40 or more SATA 3 SSDs
6,000,000,000 or more mice and keyboards