[2023] What mod is causing Transparent Walls? (Biome 4 Great Sea Only)

This looks like the "Transparent" line of sight setting. It allows you to see a dim view of the "background structures" and the exterior hull of the sub, with the brightness being tied to the amount of ambient light. I suspect one of the mods has a biome with an unusual amount of ambient light, making the view brighter than usual.

I suspect this isn't what's causing the performance issues: rendering the background at a slightly higher brightness should have no impact on performance. If the performance got worse as soon as you entered this biome, my guess would be that something else in the biome is causing the performance hit: perhaps a very large number of level objects (like some environmental hazards or things that emit light).