lokt;
Much more easily demonstrated than described, but here goes...
...after the last shot is fired, activate the cylinder latch with the thumb of the strong hand (left) by bringing it over/around the back strap (yes, you could...maybe...do it with the index finger, but the thumb seems to work better for most) and pushing forward. Push the cylinder out with the right thumb while making sort of a "V" with the thumb and fingers of the right hand...the thumb will go through the frame and the fingers will "cradle" the gun under the barrel / triggerguard area.
Turn the barrel straight up and use the index finger of the right hand to push the extractor rod and eject the empties, then turn the barrel down...muzzle toward the deck...and keep outward pressure on the cylinder with the thumb while using a finger or two to prevent it from just spinning freely...or closing part way, etc. Insert speedloader...which should be carried in a position where you can reach it easily with the strong hand, and grabbed from the belt by the "barrel" of the loader, NOT the knob...then use the fingers to twist the knob of the loader once inserted. Let the loader drop (you waste time if you throw it...gravity will work for you here) and close cylinder while you raise the revolver back to the target and resume firing. "Smooth is fast"!
You may see other lefties who have worked out a different system, and that's fine, but the method I have described has worked very well for decades and is both fast AND secure in the field. Be careful not to twist the knob of the loader as you draw it frim the carrier, as this will activate it and your ammo will be on the ground...if that happens, drop the loader and grab another...you DO have another, don't you? Ha!
Be certain to keep chambers clean so cases don't stick, or live rounds fail to chamber completely.
12 rounds in 20 seconds is plenty of time if you are smooth. 1 1/2 sec, to draw and fire the first shot, 1 shot per second for the next five, unload, reload, then one shot per second should allow you easily 7 seconds for the reload...duck soup!
Good Luck