Here it is quantified, the difference between conventional Cranksets and the Compact Crankset from http://www.slowtwitch.com/mainheadin...r/gearing.html. The following examines the speed differences between a 53-39 Chainring and a Compact (50-34) Chainring. The calculations are based on a 700c wheel with a cadence of 90 RPM. Bold print marks a change in cassette size.

...::: 700C Wheel At 90rpm :::...
53-39 Rings, 11-21 Cogs 50-34 Rings, 11-21 Cogset
High speed 33.2 MPH 31.3 MPH
Low speed 12.8 MPH 11.5 MPH
Comments Changes between each gear are the same (cssettes re the same). The tradeoff is easier climbing as the expense of a small loss of top speed. This s the lightest configuration for either set up.

53-39 Rings, 11-23 Cogs 50-34 Rings, 11-21 Cogset
High speed 33.2 MPH 31.3 MPH
Low speed 11.7 MPH 11.5 MPH
Comments Low speed now almost the same. 11-23 cassette is heaver with gear ratios wider apart.

53-39 Rings, 12-25 Cogs 50-34 Rings, 11-21 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 10.74 MPH 11.5 MPH
Comments The 12-25 cassette lowers the top speed on the conventional Crankset and makes climbing easier but at the cost of even wider gear ratios and weight.

53-39 Rings, 12-25 Cogs 50-34 Rings, 11-23 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 10.74 MPH 10.18 MPH
Comments With slightly wider gear ratios the 11-23 cogs further ease climbing with the Compact Crankset while maintaining top speed.

53-39 Rings, 12-27 Cogs 50-34 Rings, 11-23 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 9.98 MPH 10.18 MPH
Comments 12-27 is about the biggest (heaviest) cassette seen on Tri Bikes. Almost the same climbing can be achieved with the 11-23 cogs with a compact Crankset with a higher top speed, closer gearing and reduced weight.

Gear spacing...
weight...
...Closer gear spacing allows small changes in gearing, letting the riders to keep his/her cadence in a narrow ideal range.

Selecting optimal gearing for a bike needs consideration of the abilities of the athlete and the bike course (wind and hills). It is not unreasonable to have different gearing for different races. With conventional chain rings on a flat course an 11-21 cassette might be ideal. On a hilly course (Lake Placid IM) a 12-27 would be much better. With compact cranks you could use an 11-23 for both races with similar results. If you are an older or less powerful age grouper a 12-27 cassette with Compact cranks would eliminate the need for a triple Crankset (a heaver, more complicated solution sometimes used to get low gearing).