This is not a bug, this is a result of the fact that the synchronization of ownCloud in reality is really badly implemented.
First of all all of that stuff runs in PHP space in the web server, whatever you use, Apache with mod_php, Nginx or whatever.
ownCloud has been designed with the home user in mind, meaning the stuff should run everywhere. It runs ok if you only need to synchronize a few hundred files. If you need to synchronize more than that, in the range of thousands or ten thousands, the fundamental design flaws of ownCloud come to the light.
Meaning until recently it only uploaded files serialized, meaning one file after the other. Of course that's slow. Now they do parallel uploads, but that doesn't solve the problem yet of the poorly desigend sync component on the server and the poorly designed sync protocol.
And even with parallel file upload speed is sub par, and you still have the eternal problem of duplicate files en masse and whatever.
What ownCloud really needs is a standalone server sync component, running in whatever programming language, but exactly such a thing as a daemon.
As long as there is no such thing, ownCloud will always sound good in theory and be bad in praxis. This does include ownCloud 7.
If you need a fault proof opensource syncing solution which does scale well with thousands of files and runs reliable, ownCloud is definitely not the thing to use and you should take a look at the alternatives.