Just a thought about why FFg may seem to be slower, when its speed can only be measured in millionths of a second. We may be confusing primer ignition speed and speed of ignition of the main charge.
Because FFg has larger granules, ther is more oxygen in and around each granule of powder, and the heat of the flame produces will ride higher in the pan than when using the smaller granules( FFFg, and FFFFg). Try banking the prime away from the touch hole, and also make sure any powder near the barrel is under the touch hole.
If you think of a candle flame, the hottest part is at the top of the flame, not on the sides, and certainly not on the bottom. There is an air cone inside the bottom of the flame that surrounds the end of the wick( fuel source) while there is a glowing ember at the top of the wick, producing gases from the wax that is being consumed. The gases are being mixed with the air in the cone, and ignite to produce the heat above.
Priming powder has to work the same way. However, it is the powder granules that burn, and the larger grains take longer time to burn up completely, than smaller grains. Both ignite at the same temperature of heat, so lighting the different grades of powder does not involve greater time( primer ignition speed.)
But, if you have a touch hole that is NOT located
above the top line of your pan, there could be a delay in getting the heat into the main charge( main charge ignition speed) because the heat is above the touch hole, until most of the priming powder, consisting of FFg is consumed. To speed ignition of the main charge using FFg as priming powder, you can do several things. 1. Stop using FFg for priming powder, and use either 4Fg, or 3Fg; 2 Grind the bottom of the pan deeper( not likely for most pans); or, 3. Relocate the touch hole by redriling the barrel for a slightly higher touch hole liner; or 4. Bank your powder in the pan away from the touch hole so that the heat of the flame moves toward and into your touch hole.
I have done the last, and when I do it correctly, there is no delay in the speed of igniting the main charge. Just my two cents.