A couple years ago, I took a road trip from Wisconsin to Washington and mostly stayed in rural hotels on the way. I expected the internet in rural areas too sparse to have cable internet to be slow, but I was still surprised that a large fraction of the web was inaccessible. Some blogs with lightweight styling were readable, as were pages by academics who hadn’t updated the styling on their website since 1995. But very few commercial websites were usable (other than Google). When I measured my connection, I found that the bandwidth was roughly comparable to what I got with a 56k modem in the 90s. The latency and packetloss were significantly worse than the average day on dialup: latency varied between 500ms and 1000ms and packetloss varied between 1% and 10%. Those numbers are comparable to what I’d see on dialup on a bad day.
The flaw in the “page weight doesn’t matter because average speed is fast” is that if you average the connection of someone in my apartment building (which is wired for 1Gbps internet) and someone on 56k dialup, you get an average speed of 500 Mbps. That doesn’t mean the person on dialup is actually going to be able to load a 5MB website. The average speed of 3.9 Mbps comes from a 2014 Akamai report, but it’s just an average. If you look at Akamai’s 2016 report, you can find entire countries where more than 90% of IP addresses are slower than that!
Your site may not explicitly target visitors in those countries, but if we’re building websites for the World Wide Web, we ought to be more considerate of users everywhere.