The way I try to approach this sometimes is "combat archery." I like to think of how archers may have trained in medieval times, how quickly they would have to learn to shoot, and what the approach was. So rather than go on Google for all of it, I went outside and tried a few things on my own.
The first idea I experimented with was using an overhand grip, with my fingers curled around the left side of the bowstring instead of the right. That turned out HORRIBLE because it forced the arrow to swerve to the left every time, so I needed to try something else.
The goal now was to go back to using the same grip I always have, but to load quicker somehow. That has posed a challenge, but it's becoming much less of a challenge with each round (or quiver-full) of arrows.
-SIDE QUIVER: Having a quiver hang from your waist allows you to load the bow differently, because you can slide the arrowhead in between the bow limb and the string to set it in place. Then with enough practice you can pull it straight back to the string.
-BACK QUIVER: When you have the arrows on your back and want to load faster, you can do pretty much that same thing. the only difference is you're pulling the arrow from a behind before laying it on the arrow rest. Let the head fall between the bow limb and the string, and get that muscle memory down.
More Arrows
The other thing you can do, whichever quiver you use, is hold one or two arrows in your bow hand while you draw back. When you release, you can reach in and grab the next arrow, and pull it straight in.
There's a difference between practicing until you get it right and practicing till you never get it wrong.
Beorn's Longbow: Quick Loading Tips
A lot of this has been summarized in my latest BEORN video (I put the link right here) so hopefully it helps with visuals if the explanations were a bit confusing...
