However simplified the explanation I received from Sandisk was all that really matters is that out of all the manufacturers Canon does things a bit differently. Canon recommends a Low Format in camera to restore full write speed. It works, but, unless a Canon user is shooting video or long, high speed bursts they are unlikely to notice any difference. The details aren't important, only the results. The OP pointed out the results and I pointed out that it is a standard Canon procedure. If the rest of you want to impress each other with your knowledge of SD cards, so be it. The only thing that matters to me, and probably most Canon owners, is that the procedure exists and that it works.

My 128GB San Disk Pro was finally about full of mostly RAW images, so I made sure that I had transferred everything I wanted and reformatted the card in my CANON 80D, using the standard formatting option. I then shot several hundred images in the 80D. It seemed to me that the write speed to the card had degraded somewhat. It occurred to me that maybe I should have done a Low Level format, which deletes all information and data on the card, vs just the normal formatting which simply resets the file system.
So to test this hypothesis, I formatted the card again in the 80D, selected high shutter rate and set the camera to Manual. Holding the shutter down, I got 25 exposures before the buffer filled and the write speed slowed to a crawl. Then, I reformatted the card using the low level formatting option, everything else being the same, and held the shutter down until writing slowed to a crawl. This time I got 32 exposures before the buffer filled and the write speed slowed to a crawl.
So, this little experiment tells me that if you want to get the most out of a top tier high write speed SD card, you should do low level formatting rather than the quick and dirty standard in-camera formatting.
Has anyone else seen this effect?

I have never seen this effect. For a simple reason. I don't format my memory cards when full. I had a bad experience years ago and that has jaded me. I had a 4GB Card and filled it completely. I tried to transfer the files. I though I did and then format the card to be used again. They were expensive back then. I didn't do a complete transfer and lost all of my photos.
From then on, I have never formatted a card and when filled, I transfer the photos to my external hard drive. Then I label the card and store it away for future review of the original.
As for a decay in write speed this is a defect that the manufacturer should be made aware of. The presence of a file should not interfere with another file being written unto it. There may be a damaged pin which becomes more noticeable with each format or the memory card may need to be defragmented. If you format your memory cards do it carefully and according to the camera manufacturer's directions and card manufacturer's instructions. Play it safe and don't format your memory cards at all.

FWIW - I always low level format my SD card in my 80D after transferring my raw images to computer.

Have not had an issue.

I also do not use cards larger than 32 GB. I used a 64 GB card in my Nikon D7100 awhile back and the card failed (PNY) Luckily I had already transferred all the images already and lost nothing but the card.

My thought now is use 32 GB or 16 GB cards - if they fail it minimizes what is lost.

I also transfer images soon after the shoot, not waiting for the card to be full.

However simplified the explanation I received from Sandisk was all that really matters is that out of all the manufacturers Canon does things a bit differently. Canon recommends a Low Format in camera to restore full write speed. It works, but, unless a Canon user is shooting video or long, high speed bursts they are unlikely to notice any difference. The details aren't important, only the results. The OP pointed out the results and I pointed out that it is a standard Canon procedure. If the rest of you want to impress each other with your knowledge of SD cards, so be it. The only thing that matters to me, and probably most Canon owners, is that the procedure exists and that it works.