For some reason I suppose I always assumed (I'm not a web developer) that "lazy" on the web worked like low-priority QoS classes: they would simply run only after every high-priority (user-initiated) task was complete. It does seem bizarre to me that my web browser knows I'm on mains power, I have a high-bandwidth wired network connection, my CPU has many cores which are sitting idle, and it's using a wire protocol with compression and prioritization and multiplexing, and yet it doesn't want to spend a couple milliseconds downloading a small resource in the background before I'll (probably) need it.