Hey, not part of the Flirc team but I can answer these questions:
> How quickly does the IR blast occur after button press?
Using an IR camera, I could see the blast occur instantaneously (to the human eye).
> Can I hold volume up or volume down for rapid repeat of IR blast?
Yes you can, it works for all the buttons for which it matters in my experience. They also seem quicker than my native LG magic remote that uses bluetooth, ymmv.