Welcome to Android Central! You may very well have a clone or knockoff, because according to the original
Android Central review and to
Wikipedia, the S6 originally shipped with 5.0.2, not 5.0.1. So that certainly could explain why the performance is so poor. It also means that updating the system probably won't improve much, and updating will likely be challenging since it doesn't seem to be genuine.
Install and run
CPU-Z. What processor does it identify? If it's a MediaTek, it's clearly a knockoff.