Larry Pletcher has done a lot of measurement of flintlock ignition time (including some comparisons with caplock times, as I recall). His data seems to be offline at present, so I can't give you a reference to quantitative data. Results have been written up in Muzzle Blasts over the years, if you care to go searching for one of his articles.
What I remember is that caplock ignition is a bit faster than flint (from memory - if the lock time of a Mauser bolt action is 5 milliseconds, the lock time of the percussion was maybe 25 msec, and the flinter 35 or 40 msec). I know if the lock is properly tuned, with a good flint, as a shooter you can't perceive any delay between when the trigger breaks and the main charge goes 'boom.' But, there are more things that have to be in proper adjustment on the flinter: proper hardening of the frizzen, right spring tensions, etc, and also more things to go wrong (e.g. flint gets loose in the jaws, flint gets dull without you noticing, you prime too little or too much, etc).
I shoot flinters because flint ignition fits in with the historical time period I'm interested in more than percussion does, and I don't mind the extra challenge to keep everything working in harmony.