Could you please disconnect the USB drive once and import any locally available photo?
Do you see this issue then?
Also, do you sync your photos to Creative Cloud?
Discussing similar issues just a few threads down:Memory Leak possible in latest Lightroom (via Adobe CC) on W10Pro
t's a bit difficult to do a definitive test at the moment. I did copy around 300 RAW files to my internal drive, unplugged the USB drive and tried importing them locally. This seemed to be better but memory usage in Activity Monitor did continue to increase after the import had completed but I was able to commence a second import, albeit with only an additional 50 or so images. Again after the 2nd import, memory usage continued to increase but it did not reach its limit and cause LR to stop responding. To me it seems as though the memory isn't actually freed up after completion of the task, or if it is, the task is taking much longer to complete than it has in the past and LR doesn't give any indication that it's still processing any images.
It's not definitive test as the photos I shot on holiday total over 3800 and I just don't have the space on my system drive to try and import that many, (in batches).
It's taken me most of the day but I have now got all my photos into LR but I've also noticed that LR is now slow at updating changes. What I mean is, before if I slid say the exposure control I would see the image change in real time. Now it can take a couple of seconds before I can see any difference, which makes fine adjustments somewhat tricky.
As for synching, I do sync to Lightroom Mobile, is that the same thing?
I have over 50,000 images in my LR Catalogue, is that a problem? I thought there was no longer an official limit on the size of the catalogue.
I've discovered I get huge memory usage on my Mac (running El Capitan) which starts as soon as I hit 'Import...' in Lightroom CC 2015.6. Eventually, the Mac runs out of RAM altogether and El Capitan prompts me to close some applications. By this point, LR was using 14GB of my 16GB of phyical RAM.
However, if I import using a watched folder, everything completes absolutely fine. So importing itself seems to be OK.
Roll back to CC 2015.5.1 - there are many threads decribing how.
Post back if you don't find it.