At the risk of "trying to shed light where there is no darkness", I'd point out that tsvstar did a lot of funky things in his builds to make the camera work the way he wanted to.
Well, like I've been saying, there really isn't much to be different. As I've said a bunch of times, CHDK really isn't doing much in this process. It's just fiddling with the physw_status bit for the half press button, which
to the canon firmware should be indistinguishable from the physical button.
My comment on timing was mostly about differences between chdkptp, ptpcamgui and pressing the button manually. As mentioned earlier chdkptp, waits for the the get_shooting, and then releases the button immediately. I don't know what ptpcamgui does.
It's very difficult to see how the CHDK build would make a consistent difference in timing, although I guess it might have changed by 1 kbd_task cycle somewhere along the line. That seems like a real stretch to explain what's going on though.
I would be extremely surprised if CHDK build was actually the relevant factor. From my understanding of CHDK it seems basically impossible.
@blackhole:
Have you tried the latest chdkptp version? This changes the half shoot delay to 3 seconds.
Another thing you could try is putting some delay after get_shooting goes true.