Hi Jeff, Essentially if your valve clearances are out. The valves will not open to their optimum distance and its harder (slower) for the gasses to enter and leave the engine. With a diesel to adjust the clearances it is know as one of the primary ways to make the engine run efficiently.
If you have a ticking sound on idle from high up in the engine, then its possible that they need attention. If they are real bad then it can wear the camshaft.. Hope this helps..