I'm not sure if this has been mentioned within this behemoth of a thread, but the IRQ Resource of the mouse is an important consideration. Before checking this, mine was on IRQ 16, which is the same as both my GPU and sound card. While I couldn't personally change the IRQ for the GPU or sound card because there's only 1 available slot for each on my mobo, you can change the IRQ Resource of your mouse by changing the USB port is plugged into.
After I tested a few ports, I got my mouse on its own IRQ 23. This not only made the mini-spikes in DPC latency checker and LatencyMon fewer and farther between, demonstrating improved input lag, it also polls much more consistently @ 1000Hz in MouseMovementRecorder. Before there was ~2 strange polling deviations around 900-950Hz and 1050-1000Hz every 50 polling measurements during constant high speed mouse movement, and now it sticks to 1000Hz +/- 5Hz like a laser beam.
Here's how I went about checking the IRQ resource and changing it:
1) Windows key + R, type devmgmt.msc and hit enter, then View > Devices by connection. Find your USB host controllers and open them up until your find 'HID-compliant mouse' as a 'USB Input Device'. Then right-click the USB host controller that you found it under ('Standard Enhanced PCI to USB Host Controller' if you're using the Microsoft driver), Properties > Resources tab, and look for the number in parentheses at the end of the IRQ line.
2) Windows key + R, type msinfo32.exe and hit enter. Under Hardware Resources > IRQs, you can see which other devices your mouse's USB host controller is sharing IRQ Resources with. Ideally, it would have an IRQ Resource all its own, but you definitely want to try and have it on a different one than your GPU, sound card, in-use network adapter, and possibly even USB keyboard (I use PS/2 to avoid this one altogether).
3) If your mouse is on a conflicting IRQ Resource, swap it to another USB 2.0 port** and repeat these steps until you find an IRQ Resource all its own or at least on the least-conflicting one available.
4) [Optional] Tweet @AlGore that you won the internet. Laugh hardily.
**I have USB 3.0 disabled in the BIOS options as r0ach recommends.
CS:GO and TF2 sensitivity - 1 @ 850DPI, 48.9cm per 360°
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