Check any link that talks about performance. Ah, and forget about that "texture conversion" process, way too complex.
You just need:
- the reduced autogen textures (for clouds, buildings and tress -two files for those-). You'll find the links there.
- the reduced autogen variety (like, using only two or three types of tree per forest, the same for buildings).
- the FSX.cfg tweaks, just read, and we can provide more details here if you need.
