I guess it's optimized for some computers/networks but not for others. Right now I'm in an area without optical fiber so I have to use a 4g modem (12 MBps), I have an old 2013 MacBook Pro and even though my setup is far from being fast, I have no problems with most web pages, some few load kinda slow, but Facebook is in a different category, it's totally unusable, some stuff never even get to load. If I want to check Facebook I have to use Safari (the new theme is not supported in Safari yet).
New Reddit and all these JS-based websites are painfully slow for me on Safari on a modern MacBook despite being on an enterprise connection with 1Gbps and a few milliseconds ping to most of Europe. Bandwidth is not the only issue, processing all those megabytes of JS and (poorly) reimplementing browser behaviours in it is the main problem.
