Gaming

What Proc and GPU are you running? Is it a desktop? How much system RAM? OS? Just curious.

Custom scenery is likely higher texture resolution so you may be running out of System and/or VRAM running too much of it.

One app that's super cool for figuring out what part of your system is bottlenecking you is MSI Afterburner. It's a GPU overclocking app that has an on-screen display feature that shows you of course your GPU clock speeds, but also real-time FPS (like the venerable Fraps), GPU core usage%, GPU memory usage, Per-Core CPU clock-speed and usage %, System RAM usage, and more ... and graphs it all for ya too. It started as RivaTuner which has been round since like 2003, so works at least to some extent with almost all GPU's.