A common problem is to make them too tight. They should be snug, even barely loose when starting, not tight. Your foot swells in your regular shoes from just walking and standing and sitting. Imagine what it may do in form fitting shoes at 20mph sustained for hours. (yeah, I know I dream big on the speed and time, but it makes me feel better to think I can actually ride like that. )

The toe box is one of the main things to feel comfy when buying though, no doubt. When wearing steel toed shoes, they tell you that if you can feel the steel on your toes, they are too tight across there. Steel doesn't give compared to your toes. Same for a pair of shoes that will do 90+ revolutions/minute for hours against your toe area/across your metatarsals. It will feel like a steel toe cap rubbing and tearing you up. The pain transfers up your foot. Now add that you made them too tight. (cuz you didn't listen to me in the above paragraph and loosen them up, did you? )