Mac Previews Of Tiff Files Very Slow

After upgrading to Mojave, I had many issues, so I reverted to Sierra, which is able to work with CS6.
One of the most frustrating issues was Previews inability to quickly view jpegs and tiff files.
With this older OS, I am able to veiw jpegs as quickly as before, but TIFF files are still troublesome.
I noticed by accident a PSD file is processed much quicker that TIFF's, though I prefer to use the latter format for my layered files.
The tech at the store said something about newer OS's not having the same format as before, but I didn't catch the full explanation.
Why are Tiff so slow to view, while PSD files are not?