You could try to reduce the polygons and see how it would affect the texture map. I would save it as a different obj though to avoid any issues. Then I would add the new mountains to my scene and remove the old ones, copying any shaders first of course.

I've also found hiding most things in the scene works well to improve response time. That won't help with render times, but response times while working with the scene drastically improve. I'd also look at other things in your scene and look at those polys. I find trees are another source of slowdown. (I'm assuming with mountains, it's an outdoor scene) Unfortunately, those seem to need to be high poly.

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