Years of experience working with the library and being forced to rewrite almost every major component of it due to it being completely buggy and broken. Version

Years of experience working with the library and being forced to rewrite almost every major component of it due to it being completely buggy and broken. Version upgrades massively break every application using it. It's impossible to style because its "component" abstraction is leaky. The component DOM-mirror they maintain is broken by almost any 3rd party code that does anything. Debugging it is a bigger pain in the ass than debugging in IE6. The day I started using things like AngularJS and Backbone, I was able to create well over 100x more stuff in the same amount of time, all of it comparably bug-free and significantly more in line with web standards. I built a massive 1000+ view SPA in ExtJS. Getting it to scale to that point was an enormous technical feat -- it has O(n^2) and O(n^3) and worse algorithms EVERYWHERE and does massive amounts of unnecessary layout work because it doesn't actually use the DOM/CSS and let them do the things they were designed to do. The end result is everything is absolutely positioned with overlaps abound and a single page layout causes tens of thousands of unnecessary reflows and repaints. This is why you will never find anyone complaining that ExtJS applications are fast.