Slow slide between Nikon photos to Canon photos ans opposite

I have the last version of lightroom classic 7.3 (version- 10.0.16299) and i have a very slowly transition (switch) between canon photos to Nikon photos and back and forth. I have 2 files I'm working on with Smart Previews 1:1 . if I'm working only on 1 file ( Canon Or Nikon)  the switch between the photos is very fast. if I combining the canon file with the Nikon file, the switch between the photos is very slowly (1-2 second between photos)