Long, verbose variables are hard too read (it literally takes more time to read more characters), cause excessive line breaks, are harder to remember, and in general hurt readability more than they help.

Cognitive overload. Storing 3-4 succinct variable names in your head takes the same energy as storing 1-2 verbose variable names in your head.

I think there's some nuance though. If I'm the one writing the code, sure I can keep 3-4+ succint names of things I came up with in my head.

But then somebody else needs to look at this code in the future, and they're scratching their head as to what these names represent. Or somebody didn't really put enough thought into what the variables are, or gave them a name that only half represents what they actually mean.

I basically threw away all care for how long a variable name is long ago and have never had a problem. I just name it with enough words it needs to describe what it represents and don't fuss with making it smaller. With a decent IDE you don't even have to remember the whole name for things half the time, only parts of it.

Cognitive overload.. Perhaps it'd be best to provide and example of a "verbose" variable name as compared to a "succinct"

Personally I'm thinking more along the lines of

Succinct ->KEY

Verbose -> MAIL_SERVICE_API_KEY