I’m guessing that the web space is on NFS-mounted storage, while /tmp is local disk. I just tested this on my own server, which is not OVH, but is having the same problem. On NFS-mounted storage, v 1.6+ is glacially slow with file-based sessions. On local storage, it’s okay, but it’s zippier for me using DB-based sessions.
Update: Bug has been fixed in Piwik 1.10 !
Changelog Archive - Analytics Platform - Matomo
Download the latest version and enjoy (we will now auto-detect when the host runs on NFS and enable DB sessions instead!).