Correct.
> only get really important when you want to print the string.
Print, split into fixed-length pieces, render, layout, typeset — GUI apps do that a lot with their strings.
Even web browser do, probably that’s why JavaScript strings are UTF-16 despite web in general is mostly UTF-8, see section 6.1.4 “The String Type” on page 67: http://www.ecma-international.org/publications/files/ECMA-ST...
GUI rendering is very expensive for lots of reasons, but I don't believe that's one of them - even in the UTF-8 case. You're ultimately either drawing thousands of pixels or communicating over a (relatively compared to the CPU cache) slow bus with the GPU.
In modern software, GPU draws pixels. Before it does that, CPU lays out these glyphs. Because GPUs are ridiculously fast these days, the layout step is typically slower than painting.
I use MS edge browser. The built-in profiler said this comments page took 14ms to layout and only 6ms to paint. This page contains just a few tiny images, the majority of the content is text. I think the layout step spent most of the time iterating over the characters, looking up glyphs and measuring various blocks of text on this page.