This might be a placebo effect, because last I heard enabling WebRender would leave the usual renderer enabled as well in order to paper over yet-unimplemented features, which should often result in double the work and, one would assume, worse performance. But if it doesn't feel slower, then, well, that might be some kind of triumph. :P
Indeed, I've caught myself thinking "it's not supposed to render this fast" a few times myself. Really am enjoying the nightlies.