Hi guys  , I don't know if you benchmark already RIFE 4.25 4.9 .. with a tool like difference image frame by frame , but maybe I am wrong with this technique,
Anyway I compare frame by frame and the result :

magick compare -metric RMSE v1

the closer the result is to 0, the smaller the difference between two frames, and therefore the greater the sensation of fluidity.( sorry for my english I am french ) 

Result for the first 30 frames for the same video (video 1 RIFE 4.25 Heavymodel  video 2 RIFE 4.9 ) frame by frame in 60 fps


https://i.ibb.co/xKNkrFYJ/framebyframe.png

What do you think?