Tessellation quality affects performance greatly, but you can lower it without too much of a quality hit. It affects the polygon count of 3D objects and can dynamically lower quality detail of far away objects. Here's an example of tessellation quality effect on cobblestones:
(Source)
Ambient occlusion is a lighting technique that affects image depth and can improve quality quite a bit. There are two types of ambient occlusion. SSAO is a cheap method that relies entirely on the GPU. HBAO and HBAO+ look better, but are more taxing on the hardware. Because of the quality of your CPU, I'd either stick to SSAO or disable it.
Example
I never managed to fix my own load time issue with Mantle. What I noticed was that it was always initial load that was affected. For instance, travelling to the Hinterlands the first time took a very long time, but if I loaded the area a second time within the same play session it suddenly loaded quickly. I read that you could fix it by removing the Mantle shader cache from the save folder, but that never worked for me.
It's a shame that Mantle support is so bad because it gave me a spectacular performance boost on my old PC. On my current PC it actually runs a bit worse on Mantle. It's especially bad at handling the rain in the Storm Coast area.
