I am having the same problem saving Tiff files.
My files are 1GB but I have 32GB Ram. The 1st 95% saves fast but the last 5% can take 5 mins.
Activity monitor shows no other task taking up CPU % (all tasks <10%)
Every now and then the CPU usage for PS spikes to 25-50% in that last 5% of saving the file.
This is very frustrating. Same issue with a newly rebooted system and no other apps running.
The usual answer to this is to disable compression in Preferences > File Handling, but Preferences shows this to be for PSD and PSB files. It can make a massive difference saving times, so I'd be inclined to give a try even if your issue is with TIF files.
Note, we had a similar thread here a few weeks ago, and out of interest and to prove a point, I tried saving a very large file with Compression on and off. A 30 second save with compression off, turned into ten minutes with compression on, and that was with a half reasonable system.
I checked the box to disable compression, but had NO NOTICEABLE change in save times - still takes several minutes (like 10 minutes or more) to save a large PSD file (662 Mb) with several layers and a big PNG layer. I have 16 Gb RAM, dual HDDs, GeForce GT630 GPU... and this has been a very adequate system when running CS5 and 2015 - but 2018 is useless to me now.
This is so annoying it makes 2018 CC version UNUSABLE for all intents and purposes. I have tried using my second HDD as a scratch disk - no change. I tried disabling my GPU - no change. Turned the GPU back on - no change. Bottom line: I can NOT figure out why these large files take so long to save in 2018. The same files running in 2015 or CS5 worked / saved "normally" - meaning large files DO take longer, but NOT 10 minutes - more like 1 or 2 minutes at the most.
I can not understand why 2018's performance in this matter is so much worse than its predecessors. I wish I had a solution. For now, I'm uninstalling 2018 and going back to 2015 or CS5 so I can actually get some work done.