Look to CPU load and GPU load. If you will see 100% then it is the bottleneck.
Possible causes:
1. Slow memory bandwith can be the bottleneck.
2. PCIe speed can be the bottleneck.
When the RX 6800 is near 10% usage what CPU usage?
---
How to check PCIe speed. Download GPU-Z
https://www.techpowerup.com/download/techpowerup-gpu-z/
Click at "?" button and press: Start Render Test. After few seconds you will get measured speed.