Hello and welcome to SL/Forums.
Okay let's see here.
To compare, my specs are:
* Intel i7-4790K* EVGA Nvidia GTX 970 SC* 16GB RAM* SSDWindows 10 64bit
On the "Second Life" viewer v4.0.x (aka "official", "Linden Lab viewer"), I get a theoretical peak FPS of 200-220 FPS (empty regions), between 25 and 80 during normal use (depending on the scene), and an average of 45-50.
Your machine should be plenty.
So let's run down the basics here:
Clear your Second Life Viewer settings (in your case, C:\Users\[user_name]\AppData\Roaming\SecondLife), your viewer cache (C:\Users\[user_name]\AppsData\Local\SecondLife)
Install the latest SL viewer (https://get.secondlife.com)
Go to the Nvidia Control Panel and manually disable SLI (for testing purpose)
Reboot your computer after disabling SLI (it can bug out)
Without touching the graphical preferences at ALL, try the newly installed Second Life viewer in a publicly accessible place that represents the expected "experience" (like "ahern" or "Ivory prim tower") and a fairly empty region (search for "Sandbox" or try a random region on the map)
Keep track of your viewer performance using the statistics panel (control+shift+1), write down the FPS displayed while wandering around the sim, take note of minimum/maximum, and any oddities in your FPS that you encouter.
AVOID the use of "Statistics"/teamspeak/mumble/etc overlays and other "debugging" tools. They can interfer with the viewer performance. (if you don't know if you're using one, you probably aren't).
Please report your findings with FPS values associated with the regions you visited so that we have more information to work with.