Short version: playing a game on monitor A (@ X hertz) and displaying hardware-accelerated content on monitor B (@ Y hertz) will result in a substantial decrease in the FPS "feel" of the game. The FPS dip is NOT reflected in any FPS monitoring tools (Steam, Afterburner, etc). In other words, something is interfering with display output AFTER the graphics card has rendered the image.
First, I will post my current setup although it's not necessarily relevant as I've had the issue across two different builds now:
Intel i7-4790k @ Stock
Sapphire R9 Fury Tri-X (OC'd version, stock OC)
Left: ASUS VS247 @ 1080p/60hz
Center (Primary): ASUS VG248 @ 1080p/144hz
Right: ASUS VS247 @ 1080p/60hz
Windows 10 Pro (x64)
I am running the latest AMD driver package (15.7.1) and I have already tried a clean re-install of the drivers. This issue has occurred on two different Windows installations and two different AMD video cards.
Scenarios that can cause the issue:
a) Gaming on 144hz monitor, recording with OBS and have preview window panel open on any other monitor which refresh rate is NOT 144hz will result in lag. Disabling preview panel OR minimizing OBS interface fixes it
b) Gaming on 144hz monitor, opening Twitch/Youtube/Google Play Music/etc on any other monitor will result in lag. Again, minimizing the "source" app will fix it.
I first discovered the issue over a year ago when I had a Korean 1440p "overclockable" monitor, which I ran at 96hz. I observed the same behavior above, but I didn't bother asking for support since I was obviously not using the AMD drivers within spec (they have to be patched with a third party tool). However, I've since gotten a new computer, new monitors, and a new video card and the issue remains.
I did some research and it looks like Team Green has some very similar posts on their forums and it was often related to Shadowplay and was subsequently fixed in a driver update. I've googled around and seen random people here and there describing the same issue, but nobody has ever posted a resolution.
Anyone else seen issues like this? I don't expect an explosion of replies, but this likely is affecting a lot of people who run multi-refresh rate setups and they just don't realize it. It's extremely noticeable in games where the FPS rate is very high. I notice it easily in TF2/CS:GO. As soon as I disable the other content on other monitors, the game is fine again.