Dual-wielding doesn't seem to penalise the combat speed extra attack.
Upon duelling a level 16 fighter, their first AB roll with their medium-sized weapons was at a 27, 22, 17, 12 etc. When combat form triggers for their combat strike +3, their attacks are at 30/25/20/15, with offhand 30/25, however there is an extra attack made at a 34 AB.