Thanks, Arnaud. Actually, to my knowledge, John Louis is the ONLY one who can solve big cubes BLD and is actually slower than me at the 3x3x3.
I know Chris, Daniel, and Tim are not considered by most to be very fast, but they're blindingly fast from my perspective. And even you are pretty amazing at the 3x3x3 from my point of view.

(Especially when you consider you use a "beginner" method.) (And honestly, calling your version of the method a "beginner" method definitely deserves quotes, as optimized as you've gotten it.)
I guess I'm improving 1 second per 300 solves because I'm still a beginner by speedcubing standards. I'll probably see it taking many more solves per second of improvement as I get better (as Karthik suggests).
I guess I was wrong - it really does take that much practice. I'm just not sure I can see working that hard on the 3x3x3 - it's just not as much fun for me as most of the other events. But I really get hurt by my "lack of fundamentals", I think, which is why I started working on it at all lately.
Oh, and up until very recently (the past few weeks), most weeks I probably didn't do more than 20 regular 3x3x3 solves per week. I usually did more 3x3x3 BLD solves per week than I did 3x3x3 sighted solves. I guess that explains why I'm so bad at it.
Oh, and I just want to say thank you to everyone for your responses. They give me reason to believe I might be able to get reasonably passable at 3x3x3 someday. Thank you!