When I try to to open up the main menu when starting up the game everything just slows down to a crawl and I can't even use my mouse to click "New Game." I've installed the Kentie's Direct10Renderer from the HDTP Beta Release installer I downloaded from ModDB. I've also installed Kentie's launcher through the new vision installer. What's happened? :(

Just a guess but are you forcing vertical sync in the game or on your graphics card?

Personally I would skip HDTP and New Vision, just get Kentie's launcher from his own website.

I have Win8 and DX10 compliant video, and I still find better results with the original DX renderer. The DX10 renderer glitches all the time, I advise against it.

The only modification I use to the GOG install is kentie's DeusExe 5.3 launcher to set the video options, and everything works wonderfully.

Turns out, I forgot to set the renderer to Kentie's. Derp. So yeah, my fault