Highway 9 between Silverthorne and Kremmling allows you to bypass the 20mph switchbacks of Berthoud Pass in favor of Eisenhower Tunnel. Highway 9 is unremarkable by Colorado standards but you save time. I find Berthoud Pass spectacular.
While Doc Savage offers good advice on traffic, you don't get to see Glenwood Canyon between Denver and Steamboat. Using US6 between Golden and Idaho Springs will take you through Clear Creek Canyon without a clear time penalty.