As a game mechanic, I would consider a combat style trait that reduces short bow load time by 1 action reasonably balanced.
maybe it is only a very good archer with acrobatics wich uses the fast reload special effect...
if he as 4 CA and reduces loading times to 1 he could fire 2 arrows in a round and being rapid reload stackable he could fire up to 4 arrows in a round.
This videos are impressive but doing so in battle is quite another thing i don't think is so easy to hold the arrows like he does in the middle of a melee...
Fast Archery: When using a shortbow, the archer can hold up to his combat style crit range of arrows in his hand, quickly unleashing them in succession without drawing the string fully back. When doing this he does not apply his damage modifier to the shots, but effectively has a reload value of 0, so he can fire one arrow each AP. When he has unleashed all arrows prepared in his hand, it takes him 3 AP to prepare a new batch of arrows.
I like it. I would argue that not using the full bow draw actually reduces the size of the damage die (to a d4?), rather than removing the damage modifier. Otherwise the damage for an average strength archer (+0 DM) is the same regardless of if he uses the technique or not, which seems off. Similarly, a very strong archer might be able to pull deeper in the limited time before release than a weak archer. And in terms of complexity it seems equally easy to remember.
So an archer using fast archery effectively does 3d4 damage in three rounds where a normal archer does 1d6
Hmm double damage and triple the SEs against an unarmoured opponent. When you put it like that it seems a bit much.
Maybe it would be better to have Shortbow Fast Archery reduce the reload time to 1 (instead of 0) as long as the shooter has arrows in his hand. The shooter can still choose to use an SE on Rapid Reload and reduce reload time back to zero (we can assume the shooter in the video is doing this ^^ ). But then he would sacrifice an Impale or Choose Location. This is maybe more fun as the shooter has more valid options for SEs.
And against Armor 2, it's 3d4-6 versus 1d6-2: both average 1.5 damage.