Remember that although it takes 10-12 encounters to advance from level one to level two, that doesn't quite mean that you're advancing at the same rate as in 3e, because there are more levels. By the time you get to level 30, that's (assuming advancement is fairly constant) 300-360 encounters. If that's equivalent to 20 in 3e, that's 15-18 encounters per level, which is slower than the OP.