It seems that the way fog works in the nether is that the fog distance is something like:
min(6, renderDistance / 2)
Which means that even if the render distance is 8, you only get to see 4 chunks out. That means you're rendering 4x as many chunks as you can see.
There are 2 possible fixes:

• If the intention is that the nether is always more constrained than the render distance you're used to in the overworld, make the actual render distance used in the nether around half of what the render distance slider is, but keep the fog distance how it is, so that the performance is optimal without changing how it looks.
• Make the fog distance in the nether use something like: min(6, max(renderDistance / 2,  renderDistance - 2)), (the minus 2 or so to counter pop-in), as halving what's visible wastes performance by rendering a lot of chunks that you can't see. Additionally the actual render distance in the nether should be something like: min(8, renderDistance)