I got through a PhD in algebraic combinatorics in five years, but was definitely an outlier.

I had also taken and/or crashed a lot of grad classes during my last two years of undergrad (out of six, total, spread over five schools), so was probably a bit better prepared for the early years than others in my cohort. (There was also a funny system of allowing incoming students to take the first year prelims if they thought they could hack it. I aced algebra and bombed analysis in a legendary fashion, but didn't have to take the algebra test the following year as result. This /significantly/ reduced my stress load in the first year.)

Add in my undergrad time, though, and I graduated at a totally normal age...

I know several brilliant people who each spent between eight and eleven years getting their MSc and PhD in computer graphics. In theory, the MSc should take two years and the PhD should take four, but it always takes longer.

That's just absurd for computer graphics with how much has changed in the past decade.

Technology moves quickly, but is irrelevant to most computer graphics research. In practice, graphics is often similar to pure math, physics or biology. The fundamentals change slowly, and that's where academic work is usually done.

That said, it is indeed an absurd length of time. A decade is a significant fraction of your life.