Hello,
Just thought i'd report on my experience with boot times for Arm9 sysnand vs emunand. Like others, I had a typical 15 sec boot for both emunand and sysnand (large capacity card)
Funnily enough I was able to decrease my boot time by 3 secs but on my sysnand only, so now boot time is only 12 sec but the decrease in boot time is not due to moving to sysnand, the increase in boot seems to be due to having removed my emunand and re-partitioning my memory card.
So I guess what I am saying, if you are after a decrease in boot time, remove your emunand with a reparation, reformat of your mem card. Its the only thing that made a significant difference for me