Should I plug my mouse into a USB 3.0 port?
- A mouse is a slow device (the old PS/2 standard was RS232C-based), so USB1 is more than adequate.
- USB3 ports have extra connectors for the high-speed transfers, but also have standard USB2 connectors for backwards compatibility.
- Unless your mouse has these connectors (and I cannot imagine that any mouse has) it will connect via USB2 in a USB3 connector.
So you are entirely correct: plugging a mouse into a USB3 connector confers no benefit whatever. If a mouse responds slowly, it is because something else is hogging the CPU to the detriment of the mouse driver.
Just because data rate of a mouse is very low and can be handled by USB 1.x does not mean it can't benefit from being plugging into a faster controller.
There are a number of factors in play here:
USB 3.0 controllers support backward compatibility by performing enumeration in a way that is compatible with USB 1.x and 2.0 devices, and exposing a logical EHCI controller as part of the register map.
The mouse will identify itself as a USB full-speed device regardless of the port and controller it is connected to. This will cause the mouse to appear logically connected to the EHCI controller (sometimes even OHCI/UHCI).
Having a logical EHCI controller does not mean that the device is attached to a USB 2.0 circuit. The connection between system and USB 3.0 host controller will be higher speed, probably multi-lane PCIe. It's also much less likely to go through a PCIe-PCI bridge, which causes buffering and slight additional latency.
Besides the host-side connection, the transistors driving the USB pins will also have a higher switching rate, and instead of passive components for noise filtering, the bus will use digital switches in the filter network, to allow it to be removed and not slow down USB SuperSpeed devices. (This was probably a bigger deal for the jump between USB 1.x and 2.0)
The passive filter components present in the mouse should dominate. Still, rise and fall times could be a few nanoseconds faster.The registers of the EHCI controller also are implemented on the same die as the USB 3.0 UHCI logic, so the clock rate and digital performance will be higher.
Of these effects, the PCIe differences have the greatest impact on performance, especially compared to a USB 2.0 host controller connected via a PCIe-PCI bridge.
In summary, a USB 3.0 host controller with a full-speed or high-speed USB 2.0 device is not the same circuit as the same device attached to a USB 2.0 host controller, and the timing will not be identical.
However, any latency improvement will be at most a couple microseconds. I suppose it's possible that with some probability this would cause the mouse input to be relayed to the server in time to be processed one frame earlier; the probability is extremely low (well under 1%), and anyone claiming it affects their game performance is confused.
In particular, the effects of network congestion are several orders of magnitude larger. Anyone trying to give themselves every technical advantage in the competition should focus their efforts there. QoS markings probably do determine the outcome of games at the highest levels of play. Choice of service provider almost certainly does.
We need to take a look at the mouse's polling rate and from that we can have a better idea of how much data is being transmitted. If a mouse has a 100hz polling rate, it is sending data to the computer 100 times a second.
A standard mouse will send a 3 byte packet containing info on X/Y position information as well as button information. Considering that 3 bytes are transferred each cycle of the polling rate, you could have 300bps being transferred.
By default, the USB polling rate is 125hz, so by our logic, data being transmitted is 375 bytes per second.
Based on this, I don't think USB 3 is going to be any more beneficial than USB 2 or even 1.