Many people are on cell phones with low data limits. A "few MB", let's say 3MB == a few, represents 0.1% of someone's data limit. Sure, it's not that much on its own, but it adds up.
Likewise, a few seconds of CPU time is fine if you're on the latest iPhone, but if you're in a developing country on an inexpensive Android phone that few seconds of CPU time is going to turn into a world of hurt.
This cavalier attitude towards bandwidth and CPU time is outright hostile to certain classes of users.
I was objecting to the "It's 2018 and you're complaining about a few MB of bandwidth and a few seconds of CPU time?" statement, not the technical detail of JWT adding an extra ~240 bytes.
I've seen statements similar to this applied to everything from big JavaScript libraries to large "Hero Images" to 2MB GIFs embedded in pages. It's a poor argument and it's representative of an attitude that's hostile to users.
tl;dr, remember the 500ms overall budget for the humans at the end of the pipeline. No one anywhere said I want my response time slower.
Ah, yes, 2018, the Year Where Everything On The Internet Is Connected To A Cable.
It's 24 MB of bandwidth to the -server-. Your server -better- be behind a decent broadband connection, or what are you even doing?
1/3rd a kilobyte per page request to the -client-. 1/3rd a kb is a blink of an eye even on modern dialup.