Hi Robert,
Your perspective as a widget developer is valuable, thank you. I was actually just curious, because I had assumed it was bootstrap from the "look" of it with the top bar looking like a Nav Bar. Then I looked at F12, and noticed this "jimu" thing, which is hard to find out what it actually is on the Internet. In fact, I Googled and all I came up with is this company, which makes no mention of it...http://jimulabs.com/ I guess it is a framework, perhaps created solely for ESRI?
Bootstrap and jQuery are small btw. And they load async, with AMD, have minified builds, etc. jQuery file size They seem fast on my Galaxy Note 3 and i7 laptop, but I am aware those are high-spec machines. Maybe Bootstrap mulches older spec systems? I'm still excited about it though, I 'discovered' Bootstrap and jQuery last year and I am continually amazed how easy it makes things would be otherwise so hard for a GIS Analyst turned Programmer to figure out!!