Looks like the client doesn’t wait long enough. But copying should be faster than 20 min on a local disk (for 20 minutes, the transfer rate is less than 19 MB/s).
For the upload via client: Files are normally split up in chunks of 5 MB and chunks can be uploaded parallel. This increases the upload speed on normal internet connections, for gigabit networks the chunksize is too small. You can manually set a environment variable to increase it:
https://doc.owncloud.org/desktop/2.3/advancedusage.html#configuration-file