I think there's two sides to that coin. On one hand, they're slower adopting some new things. On the other hand, they've never really needed to play catch up either. I think in addition to marketing Apple are quite skilled at product modulation and planned obsolescence.
With the possible exception of the 4S, every generation of iPhone (and every generation of iPad thus far, and iPods were pretty good at this as well) has had at least one big feature upgrade. Retina, dual core, 3G, LTE, Verizon availability, size; they've been staggered across their phones in a calculated fashion. I think they could have definitely adopted a lot of those features faster, but from a business perspective, why would they? With this cycle, each year for the past five years they've been nigh guaranteed a massive amount of press, attention, hype, and most importantly cash once a year. And all the interest surely does wonders for their mindshare.
I think they could have definitely progressed their phones faster, but I also think doing so would have actually been less lucrative for them.