"Massive FPS drop" means nothing, if you can quantify that it would help rule out various possibilities. For example, if you're getting 20 FPS it's a world of difference from 1 FPS, but either would still be a "Massive FPS drop" from 60. I would ask completely different questions for 20 fps than I would for 1 FPS. Also, the link for your config has expired or been removed, so I can't look at your config to make any suggestions regarding it.

If you use X-Fire software (basically a gaming IM client), there is a known performance issue with leaving its in-game features enabled when using Inner Space. If you are using that, then the answer to your problem is to shut off X-Fire or disable its in-game features.

If this happens with a SINGLE WINDOW launched with ISBoxer, then it's most likely either going to be X-Fire or other software (try shutting things down), or you may just need to update your video drivers (this is especially true on new PCs that have drivers installed from a CD or have otherwise not had new video drivers downloaded).

Also, DirectX11 will probably outperform DirectX9 on your hardware. See http://isboxer.com/forum/viewtopic.php?f=22&t=774 for info on using dx11 with ISBoxer