New structures - Performance issues

I made a mod with new structures, walls for example. The way I did this is surely not the best way regarding the later ingame performance. I would be thankfull for any hint on how to improve the performance and where the biggest loss is expected.

  1. Exported mesh files from the Dev Kit.
  2. Imported the mesh files to Blender.
  3. Resized, combined and rearranged mesh files to a new object. (I assume this has the biggest impact on the performance. Any rules of thumb there? Like: few edges…)
  4. Imported the new mesh file back into the Dev Kit. (I imported without textures or anything, just the meshes)
  5. Applied original texture files from the Dev Kit to the new mesh. (I often used just a little part of the texture but still applied the whole thing. Is this bad and is the an easy way to cut the textures down to the part needed?)
  6. Created new collision boxes in the Dev Kit. (The new ones have way more edges, if possible its probably better to keep the existing ones)

Thanks for any answers in advance.