There seems to be a lot of problems concerning the performance of Lightroom 6.6
At the moment I am on Lightroom 6.5 standalone.
I use Win 7 pro on a 6 year old i7 870 Quadcore with 16gb of ram, a Samsung SSD and a AMD 5450 video card
I have just done a small experiment.
With just Windows running the resource monitor shows that the ram in use is 2354mb
I created a new catalogue on the SSD. Cache 100gb, video cache 10gb
The ram in use went to 2633mb
I added one Canon 5D3 image to the catalogue and created a 1920x1200 preview with high
The ram in use with no edits went to 3585mb
Now the 1920x1200 preview at 16bit should be about 13mb
So why has the addition of just one image to the catalogue increased ram usage by 952mb
Using Develop and no edits I zoomed 100% and panned the image about.
This was smooth, but the ram usage went to 4371mb, an increase of another 786mb.
This one image takes up 1738mb
Now I did some edits in exposure,clarity,vibrance,lens profile and auto upright, zoom to 100% and panned again.
Ram use went to 4606mb, up another 235mb.
That is a total of nearly 2000mb for one 13mb image.
Now I added 7 100,75,100 clones, zoomed 1:1 and repeated the panning.
No GPU the panning was very smooth and redraw was pretty instant and no CPU thrashing, which happens in my normal catalogue.
Ram uses goes up to 4919mb
That is with cloning and the above processing, ram use for one 13mb image is 2286mb.