I think the scenes overall in this game are a little more difficult than the ones in StB.
What's more interesting is that the difficulty curve is much more shallow - the game doesn't get much harder past Stage 8.
Also, of course ZUN would think the game's easy; he knows how to do all the cards :V
As for knocking the game out early - It's entirely possible to enjoy the game and beat it quickly at the same time.
If three days from now you remember most of the scenes off the top of your head, I'd say you've done well.
12-7 is easy until the last two shots. BTW, that sound you hear is Remilia crying - something about Scarlet Destiny being stolen.
---
Is it just me or does this game feel really dark?
All of the music is reserved (the lightest song is called Infinite Nightmare :V ), and there are very few light backgrounds.