Regarding the preview generator, nachoparker has a quite nice tutorial how this works and how you can optimize it:
I think owncloud has released a new media viewer but I don’t know if they generate thumbnails or if that is done via cronjobs or during the upload process. Nextcloud generate thumbnails of different size and when you access the folder for the first time they must be generated.
Did you see other performance difference? Upload or download? After the fork, the code was still very similar, so it would be interesting to see the differences after 3 years…