What is slow? How did you configure ownCloud? Did you set up caching (APCU, redis) and optimized the database cache settings? And there shouldn't be any obvious errors (check the logfile).
You could access to your server via sftp in order to measure the performance. This way you get an idea of the system's limitations (network, hard drive, ...). ownCloud probably won't match the performance of SFTP, but it's not an order of magnitude away. E.g. if you have full 100 Mbit/s network, SFTP can probably get ~10MB/s, ownCloud depends a bit on the configuration, but I guess 3-8 MB/s should be doable.