SoAlVette

Would a vacuum leak at the cruise control affect engine performance?

Frizlefrak

Yes.

Any time you introduce "false air" into an EFI engine, it affects performance. In a detrimental fashion.

rli7275

Vacuum leaks tend to have a larger effect on performance at idle and low speed. Function of percentage of volume. They also depend on the type of system. Carburetors and Mass Air systems show the effect of air/fuel ratio upset while Speed Density(Map Sensor) show the effect on idle speed (high).