1080ti - i7-6700k - 50-150fps heavy input lag, help?

is the issue specific to CSGO only? how about other games?

I had a very similar issue, hopefully following this will help you! (i7 6700K GTX 1080, upgraded from GTX 970)
Now, I am assuming you are using Windows 10, but if this is not the case it's very similar with other versions of Windows

In short: Reinstall Windows using the Media Tool and select the option that lets you keep all your data. If you don't know how to do this then keep reading, there are also some other tips that help boost FPS / reduce input lag.

OPTIONAL: Check for a new BIOS (this helped a little but it did not fix my issue, download from here - https://www.asus.com/support/Download-Center/ )

1) Uninstall your current graphics drivers.

2) Download new graphics drivers but DO NOT open/execute the file. ( Download from here - https://www.nvidia.com/Download/driverResults.aspx/137727/en-us )

3) Download the Windows 10 Media Tool. ( Download from here - https://www.microsoft.com/en-us/software-download/windows10 )

4) Open the file. ( MediaCreationTool1803.exe )

5) Accept the terms.

5) Select "Upgrade this PC now" and click "Next". Let it download a new copy of Windows

6) Accept the terms once again.

7) Click "Choose what to keep" and then select "Keep personal files and apps".

8) Click "Next", let it install.

NOTE: It may reboot a couple of times but don't turn the computer off or try to cancel it. There was a time I did this where it got stuck on the same percentage, just wait it out and it will be fine.

9) When you have a fresh install of Windows, go ahead and disable the Xbox services like usual. Disable diagnostic services and disable the "Superfetch" service, go through your privacy settings and make sure you uncheck "Allow apps to run in the background" ( If you search "Windows 10 Optimization" on YouTube, they will all most likely mention these quick tips ).

10) Run the installer for the graphics drivers you downloaded earlier. Uncheck everything besides "Graphics Driver" and "PhysX".

11) Open "Device Manager", "Audio inputs and outputs", right click → "Uninstall device" on any that you don't use. Scroll down to "Sound, video, and game controllers", right click → "Uninstall device" on any that you don't use. Often times you can uninstall everything but "High Definition Audio Device".

12) Unpark your CPU cores using CPU Core Parking Manager ( Download from here - http://coderbag.com/Programming-C/CPU-core-parking-manager-v3 ).

13) This part may be a little hard to follow, at least it was for me the first time I did it so just read and follow through carefully.
a) Hardest step, open File Explorer. Whew, so hard.
b) Go to your Local Disk C: drive.
c) Go to folder "Windows".
d) Go to folder "System32". *Don't worry, you're not deleting anything.
e) Scroll down or click "G" until you find "GameBarPresenceWriter.exe".
f) Right click "GameBarPresenceWriter.exe" → Properties → Security → Advanced → Find Owner: TrustedInstaller → Change → Under "Enter the object name to select" you will type "USERS" without the quotation marks → Click "Check Names", it will have changed "USERS" to "NAME\Users" → Click "OK" → Click "Apply" then "OK" → Click "Advanced" again → Click "Users (NAME\Users) and click "Edit" → Change type to "Allow" and under "Basic Permissions:" you will check "Full Control" → Click "OK" → Repeat that step for "SYSTEM" and "Administrators (NAME\Administrators)" → Now rename "GameBarPresenceWriter" to "GameBarPresenceWriter2"

14) Hold Windows key and press "R" to open "Run" or type "Run" in Windows search.
a) Type "msconfig" and click "OK" or press "ENTER"
b) Click "Boot"
c) Check "No GUI boot" and change "Timeout" from 30 seconds to 3 seconds.
d) Click "Advanced options"
e) Check "Number of processors" and select the highest value, for your 6700K that would be "8". Make sure everything else is unchecked and click "OK"
f) Click "Services" and check "Hide all Microsoft services" and uncheck any that you don't need.
g) Click "OK" and "Exit without restart"

15) Hold Windows key and press "R" to open "Run" or type "Run" in Windows search.
a) Type "regedit" and click "OK" or press "ENTER"
b) Follow this path "Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters"
c) Click on the Key "Parameters" ( Not the drop down arrow, the folder itself )
d) To the right there will be a bunch of different things, right click "DisableTaskOffload" → Modify → Set the "Value Data" to "0" and make sure "Base" is set to "Hexadecimal"
e) Click "OK" and close out of "Registry Editor"

16) Type "System" in Windows search and open it.
a) Click "Advanced system settings" → "Advanced" → Under "Performance" you will click "Settings..." → "Advanced" →"Change..." → Select "No paging file" and click "Set" and then you may close out of those tabs.

17) Restart your computer.

I would also recommend reinstalling CSGO. Get rid of your launch options as well. My launch options are just "-novid -tickrate 128". The only thing in my autoexec is my jump bind. Here's my Steam profile if you care to see anything else related to my config / setup - https://steamcommunity.com/id/orushu/.