FPS drop to under 30 on GTX 780 Ti

You don't have two of those 780 Ti's do you?

Sounds like what happens when I force SLI on.

Reset your nVidia Control Panel 3D Global Settings to default.  If you added the PvZ game under program settings, either remove it or default it too.

Latest drivers I'm assuming?

You could try these settings... but a 780Ti should eat this game for lunch and be hungry by 1pm.

-----------------------------------

Open Notepad.
Copy and Paste this:

RenderDevice.ForceRenderAheadLimit 1
PerfOverlay.DrawGraph 0
PerfOverlay.DrawFps 1
PostProcess.DofMethod 0
PostProcess.BlurMethod 0
PostProcess.DynamicAOEnable 0
PostProcess.DynamicAOMethod 0
Render.VSyncFlashTestEnable 0
WorldRender.TransparencyShadowmapsEnable 0
WorldRender.MotionBlurEnable 0
WorldRender.MotionForceOn 0
WorldRender.SpotLightShadowmapEnable 0
WorldRender.SpotlightShadowmapResolution 256
WorldRender.LightTileCsPathEnable 0
RenderDevice.Dx11Enable 0
RenderDevice.Dx11Dot1Enable 0
RenderDevice.Dx11Dot1RuntimeEnable 0
RenderDevice.VSyncEnable 0
RenderDevice.TripleBufferingEnable 0
GameTime.MaxVariableFps 60

Save it as "user.cfg"

You cannot have user.cfg.txt
enable "View Extentions of Known File Types" from Folder Options in Control Panel

Put it in your PvZ directory
\\Origin Games\Plants vs Zombies Garden Warfare

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Has it always done this for you?  Or did it just start recently?

Thanks for your professional respond WootCandy.

I have one GTX 780 Ti installed, so I don't know about an SLI-configuration problem atm.

I use Nvidia's GeForce Experience software, so my graphic driver always is up-to-date.

I haven't added the game to any software that can optimize it or change it in any way.

This FPS drop starting right from my first game, and I'm unable to play it now because of this.

I reset the Nvidia 3D settings, updated Windows 8.1 and did a normal reboot after. Now it runs smoothly.

Thanks for your support and time.