Oh sure, building an isolated Qt instance is quick. But adding the Yocto overhead (entire bootloader, kernel, and then fetch/decompress/build all the packages)

Oh sure, building an isolated Qt instance is quick. But adding the Yocto overhead (entire bootloader, kernel, and then fetch/decompress/build all the packages) takes a bit more time. And Qwebkit isn't trivial either. Once Yocto has it all in sstate-cache then it's only like 15 minutes for a total rebuild.