When you use a bit of inline <svg> and you don't set height and width, but you do set a viewBox, that's a fitwigoo. I love the name.
The problem with fatwigoo's is that the <svg> will size itself like a block-level element, rendering enormously until the CSS comes in and (likely) has sizing rules to size it into place.
It's one of those things where if you develop with pretty fast internet, you might not ever see it. But if you're somewhere where the internet is slow or has high latency (or if you're Karl Dubost and literally block CSS), you'll probably see it all the time.
I was an offender before I learned how obnoxious this is. At first, it felt weird to size things in HTML rather than CSS. My solution now is generally to leave sensible defaults on inline SVG (probably icons) like height="20" width="20" and still do my actual sizing in CSS.