Somewhere in Bill's comments over the years he made a statement to the effect, (which I have heard all my life), "an engine with loose valves will idle great but will not have as much power on the top end as when they are adjusted correctly". And too tight valves will cause poor performance all across the operational range. Some of the rest of Bill's comments along this line was that too much clearance caused excessive valve train wear and noise in the long term.

Back in the automobile days of old with mechanical lifters, points, and plug gaps, sometimes, when you "Tuned" the engine up and closed all the gaps up to minimum specifications, the engine didn't idle as well as it did before you worked on it. I guess this is a poor example because in this case it was more related to the smaller spark gap in the plugs?

In drag racing we would experiment some with the clearance on the old Chevy & Ford cams and make a couple runs to see if we made it better or worse.