My experience with jelly dolls and show friends only has shown more lag in busy places with jellies than show friends only feature. I used show friends only alot in FS and think it would be good for Catznip tho idk how much effort and resources it takes for such a feature to be added. Also, idk if it would effect Catznip the same way, but..
Overall I'm very happy with Catznip and it's very light compared to other viewers I've tried. Even if no more features are added to it I'm still very happy with it, thank you for making a quality viewer.