Open C:\Program Files\Steam\steamapps\common\Rocksmith\Rocksmith.ini and try with these settings:
If there's audio crackling when you start the game, increase MaxOutputBufferSize by multiples of 8 or 32 (288, 320,...). If there's no crackling you might want to decrease that number even more (most high end PCs can handle 152), but with these settings audio lag is virtually unnoticable (about equal to audio lag my XOOM effect pedal makes).
It's worth noting that MaxOutputBufferSize setting of 0 isn't what it looks like - it actually defaults to 1024 which produces noticable audio lag.
While you're tweaking MaxOutputBufferSize values, if there's crackling on the loading screen you can just kill the game with Alt-F4 and try again with a higher setting.
After upgrading to Windows 8, I had to increase MaxOutputBufferSize from 152 to 176 i.e. if you plan to upgrade to W8, expect some slightly worse performance in Rocksmith.