Using opposite aileron and rudder will give you a slip, not a crab. Slipping will cause you to lose altitude faster and can keep you on track in a crosswind, but it's best done with your thumbs, not a mix, because to do it right you have to use constant adjustments, there's no fixed percentage that will always work right. Furthermore, most planes will stall at a higher airspeed when slipping than in coordinated flight, and if you stall when slipping you'll probably spin. The way to slow a plane down is by raising the nose. Then use your throttle to control altitude, and raise or lower the nose to reduce or increase airspeed.