First find out what the valve lift should be. A Haynes manual will probably list it under engine specifications. Using a micrometer, measure a camshaft lobe at 90 degrees to the lobe. Then take another measurement over the lobe itself. The difference between the two measurements should tally with the figure given in the book for the the valve lift. Anything less, means the camshaft is worn, which can affect the valve timing, and restrict flow through the vaves. A few thou is unimportant, but any significant difference will have quite an effect on performance.
Wear is only one reason that could cause the valve clearances to be excessive, but from what you say, it's definitely one I would consider, and fairly easy to check. Mike.