I have to admit, when I 1st tried 3f in the pan it did "seem" slower. But, this could have been in my head because I expected it to be slower. Also, I filled the pan to a point that visually looked the same as when I had used 4f. When I used half or less of that amount (again visual judgement so, fallible) speed of ignition did "seem" to speed up.
I do still seem to notice that using a lot of 3f seems slower and more likely to hang-fire, but when I use just enough to coat the bottom of the pan,,, I can't perceive the difference.
Maybe some who think they can perceive a longer delay with 2f or 3f prime, are using too much?