You can't use a standard SCP build, at least, not for long. Many of the ships in Inferno have poly counts that are above the limits. You should go to the Source Code Project forum, and from there to the Recent Builds subforum. Click on the "CVS Builds" topic and then download an inferno build of the SCP executeable. Extract this and select it with the launcher. Once running Inferno R1 (patched) with an Inferno build, hopefully you'll see some improvement.
EDIT: By the way, the archive you download will have two EXEs. One is a release version, which will have the letter r on the end of the filename before the extension, the other is a debug build which has a d before the .exe. Use the *r.exe version.