one cylinder running colder than the rest.....

[Dave Rifkin]

About a month ago I had my car at a new mechanic to see if he could get the car to idle at a normal speed (it had been idling around 1000 rpm; previous mechanic claimed the car had a vacuum leak)
Anyway, he was able to get it to idle around 750 but, told me that one of the cylinders was running about 100 degrees cooler than the rest.
Would this affect the idle characteristics of the car, what could cause this and how badly would it affect the cars performance?