It got slower while I optimized it
I added lazy loading to every image on a site in about 20 minutes, felt pleased with myself, and then measured it and found the pages were taking longer to show their main content than before I started.
Blank on purpose
A client's homepage sat blank for a second and a half before it painted anything, and the code holding it there came from the tool they had bought to make the site convert better.
Measuring a consent banner
A client came back on the consent banner we'd built to ask whether the accept rate could be higher, and it turned out to be the only number anybody asked me for on the entire project.
Everyone was Susan
A full-page cache took the store's category pages from about 2 seconds to 300 ms and broke four things, and all four turned out to be one piece of the header being rendered in the wrong place.
What the CMS does to your hero image
I got a homepage's hero down to 180 KB in January, it's 2.4 MB now, and the only thing anybody did was change the picture through the form that exists for changing the picture.
The 400 KB hero
Every conversation about page weight goes straight to minification. Meanwhile there is a 2 MB image on the page that nobody has opened since Photoshop.