Then: Page was slow because the delivery was via a 56k analog modem.
Today: Page is slow because the average page size is 2.2mb. Plus JavaScript parsing + execution time. Plus asynchronous calls to go load more stuff. Plus time to re-layout around the new stuff.
I work for a large health care company. They got on the JS bandwagon a while ago. We used to have huge sites that loaded fairly quickly (under 2 seconds) and cleanly. Now? The last two years doing accessibility work - a lot of these same sites which have been converted to Angular or React? They're taking in excess of 15-20+ seconds to load.
I think it's mostly blocking chains of multiple networks requests that execute in sequence (taking a network round-trip each time). I inherited a slow ~2mb frontend app at work, which seemed horrendously bloated to me. It turned out to be not that easy to reduce to the size (due to large amounts of code depending on large libraries), but it also turned out to be possible to make it load fast without reducing the payload size.
- Every single database read depended on it
- Many of them were subscriptions rather than simple reads, and so couldn’t be trivially replaced by an API call.
Don’t use firebase guys.
You can see the size of each bundle in here: https://firebase.google.com/docs/web/sdk-metrics