EVERYONE (not just budget airlines) wants you there 30 minutes before take-off - more if there's checked luggage involved. It takes 20 minutes to board, so it's fairly reasonable to have 10 minutes for getting through security and walking to your gate.
The only time where you might get away with arriving 20 minutes before take-off is if there's no security line and your gate's right near the entry to the departures lounge.
The only time where you might get away with arriving 20 minutes before take-off is if there's no security line and your gate's right near the entry to the departures lounge.