I am using LR Classic CC v.8.0 on an iMac with 32GB of memory running macOS High Sierra. I have closed everything down except Mail & Safari and LR has used up all the memory except 5%. LR then stops completely and I have to force it to Quit. I relaunch it with everything as before and I then have 73% of my memory available and LR works smoothly for some time. After 10 minutes placing photos on the map the memory available has gone down to 21%. While compiling this message the memory available has gone down to 7%. Is this LR or macOS? I suspect LR as I do not have this problem with any other application.
Check that Lr isn't trying to build a face detection database or sync photos from to the cloud?