As you can see from Kaeko's extensive SP testing, there is a 4x multiplier to SP for ranks 1-6

http://kanican.livejournal.com/52445.html#cutid1


This multiplies gradually decreases, and you only start to see normal SP gains starting from rank 24.

Honestly, it's kind of ridiculous that the lower ranks have such high SP gain...it's rather unnecessary.

Have the journey from rank 1-15 take at least 1 to 2 hours or so, not 20 minutes, lol.