The factors affecting opening speed are motor torque, motor rpm, reduction gear ratio, and connecting lever distance from the door pivot point. You will need to measure, or estimate accurately, the torque needed to open the door (against any closing force exerted by a door-closer/spring) at the required speed. If you know the lever-to-door-pivot distance you can then calculate the force the lever must exert. From that you can calculate the torque the output gear must provide. Dividing that by the gear ratio of choice gives you the torque the motor has to have, and with which info you can search datasheets to find the right motor.