@cyperghost
Thanks for the hints !
I just tested seafile. It doesn't seem to have recurrent tasks that load the system like owncloud.
But nearly everytime I go on a page of the UI, python takes 100% of 1 to 2 cores during 5-10s, which is the time the page takes to load on my local network (withe empty repository - sic).
Moreover, unlike owncloud, seafile doesn't allow to mount and use various external storages (including SMB, S3, google, ...).
It's an interesting software, pretty easy to configure, but the more I test, the more I think the RPi kind of lacks power for this kind of applications, especially if it already hosts retropie.