Note: O'Reilly is no longer promoting the book, so I've posted a copy of Web Page Size, Speed, and Performance on my site if you're interested.
Head over to HTTP Archive and check out their trends graphs. Notice anything? That's right, web pages keep getting bigger and more complicated. The problem here is that — particularly in e-commmerce — consumers of your site expect it to load and become interactive quickly.
Turns out, the problems involved and solutions needed to fix them depend, mostly, on straightforward, Web Dev 101-level knowledge and technology. I recently wrote a short e-book on the subject for O'Reilly, Web Page Size, Speed, and Performance (free, registration required).
There's also an accompanying blog post on O'Reilly Radar, It's time for a web page diet.
Interesting side note: mobile web users (and probably app users as well) expect faster interaction than on the desktop and are more likely to ditch an action if it's slow. Think about the implications of that complication on your app development plans.