Everybody has heard the common trope: great programmers are 10x more productive than average programmers.
Then everybody heard wrong.
A 2nd edition of Peopleware summarises it; the 10x programmer is not a myth, but it's comparing the best to the worst; NOT best to median. It's also not about programming specifically; it's simply a common distribution in many metrics of performance.
The rule of thumb Peopleware states is that you can rely on the best outperforming the worst by a factor of 10, and you can rely on the best outperforming the median by a factor of 2.5.
This of course indicates that a median developer, middle of the pack, is a 4x developer.
Obviously, this is a statistical rule, and if you've got a tiny sample size or some kind of singular outlier or other such; well, we're all adults and we understand how statistics and distributions work.
Peopleware uses Boehn (1981), Sackman (1968), Augustine (1979) and Lawrence (1981) as its sources.
[ "Peopleware", DeMarco and Lister, 1987, p45 ]
That all said, if anyone's got some more recent source materials (a bit more solid than Mr Expensify's source of some guy telling him in college some time), let's have them.
Addendum: "Performance Improvement Plans" are a papertrail. If you ever get put on one, the trigger has been pulled already. Even if it seems to go well and performance targets are reached and the threat of firing is off the table, your cards are marked and your days numbered. Know that, and start looking for your next job.