Just to lay this unlockable stuff to rest.... I just unlocked everyone except seth (still need to beat arcade with the rest of the characters)
Here's what I did so it can be confirmed.
Set difficulty to easiest, 1 round matches, 30 seconds
Then unlock Sakura, Cammy, Dan, Rose, Fei Long, and Gen
Then I selected Ryu (can probably pick anyone though) and played thru arcade to unlock Akuma. I had 2 perfects and beat Seth without continuing and Akuma showed up on my first try, I beat him and he unlocked.
Next I beat arcade with Akuma on the same settings, but all you have to do is beat it. You can continue, no perfects or anything needed.
I selected Ryu again (cause his Ultra is easy for me to pull off) same settings still. 2 perfects and 3 ultra finishes and defeated Seth without continuing. Gouken showed up on my first try, beat him and he unlocked.
TL;DR version:
-settings set to easiest, 1 round, 30 seconds
-unlock all the 6 "normal" characters
-Pick a dude and beat arcade with 2 perfects with no continues. Beat Akuma to unlock him.
-Beat Arcade with Akuma.
-Pick a dude and beat arcade with 2 perfects and 3 Ultra finishes and no continues. Defeat Gouken to unlock him.
-Enjoy online and forget about the cheap AI unless you want achievements!
*EDIT* Do this only if you want the unlockable characters as fast as possible. It took me about 2 hours to get everyone cept Seth. This info is in the OP. I'm not taking credit for it, I'm just posting so there is a definite number for perfects and ultras etc.