Most people who are attending are probably still travelling from work. I doubt many would get there early on a weeknight.
5 minutes before kick off is about right for league games with our pre match entertainment. Tonight make that two minutes. That you were there 40 minutes early you must have been first to take to your seat.
usually get to games about 45 mins before kick off - get to have a rubbish pint and a natter with my dad.