One summer, a few years ago, a private R22 owner at where I used to work started to moan about his engine being down on power. This went on for a while; engineering checked the engine and no problem found.
Finally, I was asked to do a flight test with the chief engineer. When we looked at the main rotor blades they were absolutely filthy with an accumulation of dead insects, pollen all set in gooey hard stuff.
When we flew it, it did seem a bit slow, and a height climb test showed it way down on power. R22s often struggle to pass this test; sometimes an engine just seems to be low on power from out of the box.
We then got the blades cleaned and hey presto, an extra 5 knots cruise speed!