PBR was re-implemented as a fully path traced model in X16. In short, a path traced implementation more accurately calculates lighting in a scene by simulating consecutive light bounces via ray tracing. Simulating these additional light bounces do require more rays to be cast than the prior implementation, as @Renerabbitt pointed out, which does have an impact on performance. The new path traced model results in higher fidelity renderings and resolves one of the most common complaints from the prior implementation, light bleed. While reducing the bounce counts may improve performance it will negatively impact the fidelity of your renderings. Setting the opaque bounce count to 1 and checking "Approximate Additional Bounces" should essentially get you back to X15 quality and performance, but will reintroduce common issues like light bleed.
As @Renerabbittalso mentioned, another option to improve performance is setting the AMDFidelity super resolution to a higher value. AMDFidelity super resolution renders the scene at a lower resolution and then upscales the result for presentation. This approach, using the default bounce counts, will likely result in higher fidelity renderings than reducing the bounce counts and enabling "Approximate Additional Bounces".