I guess everybody has their own preconceptions of what's wasteful.

I grew up in the era of 14.4k modems, so I'm used to thinking that network bandwidth is many, many orders of magnitude more scarce and valuable than CPU time.

To me, it's wasteful to download an entire image over the Internet if you can easily compute it on the client.

Think about all the systems you're activating along the way to download that image: routers, servers, even a disk somewhere far away (if it's not cached on the server)... All that just to avoid one pass of processing on data you already had in RAM on the client.

"Mips – processing cycles, computer power – had always been cheaper than bandwidth. The computers got cheaper by the week and the phone bills stayed high by the month." - The Star Fraction, 1995

I have the same perspective regarding bandwidth, but I also consider any client to be running on a computer at least ten years old and at least three OS revisions behind.

I like to consider myself a guest on a client CPU, GPU, and RAM. I should not eat all their food, leave an unflushed turd in their toilet, and hog the remote control. Be a thoughtful guest that encourages feelings of inviting me back in the future.

Load fast, even when cell coverage is marginal. Low memory so a system doesn't grind to a halt from swapping. Animate judiciously because it's polite. Good algorithms, because everyone notices when their cursor becomes jerky.

Okay but how do you compute an image? How would your browser -- or any other client software -- know what's the hero image of a blog that you never visited before, for example?

I feel like I am missing something important in your comment.