And pngslim turns some of them into 3-5 KB files without changing even a pixel... by the time I saw more than a white blank screen, it had already downloaded 2 megabytes, with no end in sight. That's just nuts.

Everyone, always: If you make websites on a super fast connection, get a proxy (or VM) to simulate a slow connection...! You will learn a lot of things which will become easy habits quickly, and then you can turn off the proxy (turn it on again occasionally).