It's possible your monitor has G-Sync (https://en.wikipedia.org/wiki/Nvidia_G-Sync), and not FreeSync (https://en.wikipedia.org/wiki/FreeSync)

Nah, it's an old benq G2222HDL, without either of those

This might sound weird, but do you have the BenQ DVI driver installed? [1]

[1] https://www.benq.com/en-us/support/downloads-faq/products/mo...

Interesting. To be clear, the only change was the GFX card (and driver)?

There is a minimal chance that I might have used the VGA port on my monitor, and now I'm using DVI.

I'm like 80% confident I've used DVI in the previous setup, but there still is a chance that I'm wrong.

I'll probably try getting a dvi->vga converter in the next couple of days and see if this is the culprit.

Can you do a display port cable instead of either of those?

Monitor only has DVI and VGA

Do you only notice the input lag in-games or does it persist when, say, dragging application windows around?