Its just like that Multi Res mod that made for Diablo 2 v1.12a when you are seeing areas that you don’t supposed to see, you get frame drop issues.
In River of Flame area if you get close to lava ocean you will get this issues but if you stick to the middle of the road and avoid the far ocean areas you won’t get this issue.
I remember many other places have this issue in old diablo 2 with high resolution mod. And bug section already filled with frame drop issues.
I don’t fully understand the issue but i know its related with seeing outside of gameworld. This is not easy to fix. The only solution for D2MultiRes mod was lowering the resolution.
Maybe they can add forced zoom some areas to prevent this .
