If you own L.A. Noire on PC and aren't a dinosaur, you probably have it running on a Windows Vista or 7 configuration with a DirectX 10 or 11 GPU. Consequently, it seems you're not seeing the performance you should, as addressed in today's patch.
Rockstar has added DirectX 11 support to its detective game and noted it improves performance for those will DX11 GPUs. With that there's an option to switch between DX9 and DX11 at your leisure.
We can't speak to the graphical enhancements as we don't actually have the game, but feel free to share your own impressions below.
Now if there was just a way to remove that 30fps cap...