How tight should cycling shoes be? Ski boots are notoriously difficult to fit. The fitters will advise to go one and often two sizes down from what would at first seem like a comfortable fit.
Do you find any performance advantages from wearing tighter clipless cycling shoes? The sizes on Shimano shoes seem to be one to two sizes too small from what one would expect. Is the reason for that that they are meant to fit (very) tight?
Update: socks
Companion question: What is the thickness of the socks you wear with cycling shoes? I imagine cotton is a bad idea in an intense (i.e. sweaty) workout. Polyester or wool are the only two options (moisture wicking, less hospitable to bacteria growth, etc). Wool would not help much with guarding against cooler temperatures, 1- because wool socks are usually so thick they'd require a different size of shoes, 2- because the shoes are ventilated and so the inflow of cool air will anyway negate the advantage of wool. Hence polyester is the only option.
Can you confirm in your answer that the thinking above is accurate, that cycling shoes are worn with polyester socks. These, I assume, would not modulate temperature. The socks one would wear would remain on the thin side. One would modulate the temperature by full leg covers (and shoe coats if/when necessary).
Briefly: Can you comment on the thickness of socks that one would typically wear during summer? Does it boil down to preference? Some people wear soccer shoes with extra thick socks and others with thinner socks. Does the same apply to clipless cycling shoes, or does everyone eventually find that thin or thick socks are the way to go?
In my experience you mainly want them to be comfortable.

But there are several considerations:

If the shoes are too short your toes will bump the ends, and this can become torturous 75 mile into a centurion.

On the other hand, if the shoes are loose your feet will slide back and forth.  Not only can this be painful, but it means that you will not have a steady foot on the pedal, but rather the relationship between the ball of your foot and the pedal's center will be constantly changing, resulting in poor pedaling (and some additional pain in the foot).