I bought one of the first GSync monitors many years ago (ASUS PG278Q ROG Swift) and it goes from sleep to displaying an image in just a fraction of a second. It's totally amazing, way faster than any other monitor I've ever seen, including newer GSync monitors. It only has one single input port though. I've never tried hooking it to a KVM, but I bet it would switch super fast.

Maybe try one of the first gen (2014) GSync monitors along with a DisplayPort KVM, if that can work for you. It's my understanding that the first gen GSync monitors all used the same monitor driver board that Nvidia made, so they probably behave similarly.

Honestly I didn't know how much I needed fast wakeup until I had it. It's a way better feature than GSync, yet monitor and TV reviewers generally ignore it completely. There are probably newer monitors with fast wakeup but it's impossible to know without testing. My TV takes like 8 seconds to even turn on its backlight and it's just ridiculous.