New render
0.3 Detail
3 AA
1/1 GI
rendertime: 11min
Try it with a render detail of 0.7 The higher settings are worth the longer render time on this one.
The difference is very minor TBH, though i think detail was 0.4 in first render.
Since most geometry (for the rasterized renderer) is in the background it can be correct that you won't need very high detail settings for good results.
However, GI relative detail, as the name says, is relative to the detail setting.
Therefore you might not need detail 0.7 or higher for your terrain, but it will definitely have influence on the accuracy of GI.