P: Noise reduction effect not displayed in Develop mode

Try this with your 5D images... use the slider in Develop to zoom 1:3, then 1:2. Can you see the NR? The point at which you can see the effect is the minimum zoom at which NR is visible.

Your 5D files are 6700px wide and your display is 1920... that's why you can't see the noise reduction. Noise is 1px wide and when you fit, you're only seeing 25% of your image. There's no way to see NR at that scale... that's why there's a viewer in the Details panel.

You can't see noise reduction if your images are only displaying at 25%. Noise is a per pixel change in an image and there's no way to see how reduction works unless you're at 100%.  

Tried different images. Cropped file to 2606 x 1737 - NR displayed correct in FIT. Cropped file to 4386 x 2467 - NR not displayed in FIT.

UPDATE: So I've trying different things and here is what I see,

1:1 vs 1:2 vs 1:3 vs 1:4
NR results shown in 1:1 1:2 and 1:3 but not in 1:4.

Camera Calibration
There are 4 process version under camera calibration. The current version is 4 and I am guessing the previous version(before updating to LRCCC) was 3. NR results shown in only version 2. Not even version 3 😐

Moving sliders very fast 🙂
When I move any slider(even HSL sliders) real fast, when the slider is moving, the NR results show. Then it almost instantly goes away. Its almost as if it is being rendered to look crappy.

Camera Raw
I tried RAW files from my 5Dm4(30mp), X-E1(16mp) and 70D(20mp)....guess what...the 70D files work just fine!!!! This is really frustrating cos I only upgraded to the 5dm4 about a month ago.
 
As for the Fuji, I usually don't shoot raw with my xe1, so dont know if this issue was present with the Fuji RAW before LRCCC...maybe its an xtrans thing. Then again, the 5dm4 RAWs may be weird because of the Dual Pixel Raw option(although i am not using DPR)

Anyone else seeing the above?

Please please tell me that people with this issue are not 5dm4 users. That would almost be funny...unfortunate, but funny.

All my drivers are up to date. I'm using windows 10 and GTX driver 388

Tomorrow m gonna hook up the PC to the TV(4K) via hdmi(no DVI) and see what happens.

On a side note, my monitor is 1920x1200 and I changed resolution to 1920x1080 + restart. Lightroom wont even open. It just kept crashing.

Instructions did not work for me. Setting GPU preferred system = "OpenGL" changed "Metal" to "OpenGL" in Performance tab but noise reduction effect not displayed anyway. macOS 10.13 (completely updated), GT650m.

Just to update, the latest Nvidia drivers came out today, ive installed them and the same issue so thats 4 sets ive tried now.
Im firmly convinced this isn't a driver issue - its a lightroom issue.  Especially as it effects both windows and apple (which is rare).
Its also not just noise reduction - its sharpening not updated as well (but thats harder to see on fit and fill than NR is).

When applying noise reduction to an image you can only see the noise reduction tale effect if your zoomed in to 1:1. When you zoom out again the noise reduction is not applied.

No problem for me, tried several Sony A7(R2) raws with Lightroom Classic CC on macOS Sierra 10.12.6, pulled the Luminance slider back and forth between 0 and 100 % and clearly saw the effect in the "Fit" view. I applied this fix (marked as official for a different issue).

I've got the same issue, but I'm on a MacBook Pro 15" Late 2016 with macOS Sierra 10.12.6

I can't see how it's a driver issue if it's across macOS and Windows?

Turning off the "Use Graphics Processor" option resolves the issue.