Initially it'll lower the nose and descend in a shallow dive to accellerate back to the trimmed airspeed. But that trimmed airspeed will now occur at some amount of descent angle. The extra 1/2 lb requires some additional energy to hold it up. That energy has to come from either the power of the engine or from converting the potential energy of altitude over to kinetic energy by "gliding" downward at a rate that produces the additional energy needed to support the extra weight.

To return to level flight one of two things can happen. One option is the engine can be throttled up until the extra power allows the model to return to level flight at a higher airspeed that allows the wing to generate the lift at the original angle of attack. The angle of attack for this particular level trim being set by the elevator. The second option is to ease back on the elevator to return to a now slower level flight speed at a higher than previous angle of attack. This second one works because we're making the wing work at a higher lift coefficient. The conservation of energy is still working on this since you've traded some of the airspeed for higher lift so now more of the engine's power is being used by the wing for lifting and less is going towards fighting parasitic drag. What suffers in this elevator trim option is the flying speed.

You were quite correct when you countered the idea that a heavy model isn't slower but a heavy model IS more draggy than a light one. When the person that said a heavy model is "slower" this is likely what they meant.