I would think that if you were consistent with your selections, then you could develop your line to hatch in 21 days instead of 22 or 23.
But the real question is should you do that? Realistically, culling can select for anything, but it cannot select for everything at once. If you had a common breed of chicken (i.e. RIR) that is already commonly bred to the SOP, and you had a line with prolonged incubation, then selecting to correct that one trait wouldn't "cost" anything because the breed is already up to the SOP and readily available at that level. On the other hand, Dorkings are rare, and even the best ones are still not quite to the Standard. So if you select for something like incubation time, then you might eliminate from your breeding program a longer-incubated bird with excellent conformation or size or temperament or vigor or egg production or meat tenderness or leg length or etc,etc,etc, -- all of which contribute more towards reaching the goal of breeding a Dorking that finally meets the SOP in every way.
So can you reduce your incubation time, yes. But unless it's adversely affecting the chicks in some way (i.e., reduced survival, poor vigor, need for assistance at hatch, etc), then I wouldn't think it would be worth the "cost" your line would pay in lost opportunity to select the best overall birds from among ALL your chicks, not just the 21 dayers.
i'm not sure it's something 'fixable'... just hatch everything you can and keep the best for breeding.
Thank you both so much for your input. This really helps. It is so nice to have you all here when I have questions. I'm sure ther will be many more to come