The renderer's pixels and egui's are both premultiplied RGBA8, so color_image copies the bytes in one go. At 3456×2234 that takes 0.9 ms instead of 3.4 ms per update, at 5120×2880 1.6 ms instead of 6.8 ms. Every rendered image the UI shows (canvas, thumbnails, previews) goes through it.
Its glyphs were put in visual order and then cut where the path ran out, which kept the text's end. Now the longest start of the text, in reading order, whose glyphs fit in visual order is laid out, as area type does with each line it breaks, by the same rules that place the line: steps where the glyphs sit, hanging punctuation and the path beyond the brackets.
When two or more frames stayed in the thread, the first got the whole story while the others kept their shares, so the re-flow joined those twice. The frames after the first now start empty, and the story flows through them once.
Illustrator links type in an area or along a path (https://helpx.adobe.com/illustrator/using/manage-text-area.html); until now only area type threaded here. Type on a path that overflows ends its line where its text stops fitting, so a story flows on from it, and its red out port, just past its end bracket, loads the Selection tool with the text as area type's does. A new frame threaded from it at a point is type on a path of its shape.
Jürg Lehni