Vello is intended more for general vector graphics and would probably perform better with pictures containing lots of large paths. Slug is designed specifically for rendering glyph-like objects and would perform better with lots of text and icons.

im not Eric but: having used vello (when it was in its early stages i "transplanted" it into my game engine), it is quite a beast. its got a multistage compute shader pipeline, and over time im kinda soured on the whole idea

vello will probably do great under very heavy loads but for your average UI or text document? i reckon something far simpler (like slug!) will end up being more efficient simply by avoiding all those shader launches