I've always wondered about this.
I mean, back when firefox was available in gtk1 format I always used it instead of gtk2, simply because it was faster.
File-roller takes a couple of seconds to load, while gnozip loads instantly.

Would it be possible for GTK2 apps to become as fast as GTK1 apps? What extra features does GTK2 have which makes it slower? I certainly haven't noticed anything...

(I'm not really sure of the proper names of GTK1 or GTK2, but hopefully you know what I'm talking about.)

GTK2 is newer and look a whole lot better Some of the differences are better theming support, GTK2 renders using Cairo which is a vector library although at this stage I believe it's marginally slower. Not sure about the final release but when they were testing it there was a speed decrease. They may have resolved that issue though, I'm not entirely sure. GTK2 supports more widgets out of the box.

If you used a lighter theme with no pixmaps and no exotic render engines then you could probably get it to be a bit snappier but (this is subjective) comparing Arch Linux packages, GTK1 is a measly 1.7MB compared to GTK2 which is 5.7MB, there is clearly a lot more stuff in there which wouldn't contribute to speed.

Heh, I forgot to mention I also prefer the themes of gtk1... Is there any way to make the buttons smaller in gtk2?

i think depend on your purpose that you would use which GTK is

All of you are English speaker so GTK1 is OK for you.
But GTK1 doesn't support unicode fonts. If you are a non-English and you want to see your native to be displayed correctly, you must switch to GTK2