Owncloud supports scale-out model for using multiple servers:
https://doc.owncloud.org/server/8.0/admin_manual/operations/scaling_multiple_machines.html
But the primary question above is what your bottleneck is. Depending on where the vast majority of load actually happens, you can do quite a few things:
- Is it network-bound, specifically slow with uploading smaller files? To my knowledge, owncloud sync files one-by-one; having more than one servers won't help you with that. One thing you can do about this, is hooking your existing hard drive directly up to the server, and doing a local sync first
Lower-probability problems:
Is it DB-bound? (check: mysql CPU usage on server) If so, you can move the mysql server to the other computer, and run it from there.
Is it server-application-bound? (check: PHP CPU usage on server) If so, you can set up a load-balancing between the two servers, install php+owncloud on the second one, point it to the first one's DB instance, and reduce the load.