1. lots of transparency with the GPU acceleration on - it very quickly becomes an order of magnitude slower than the CPU renderer, especially if you do tricks to generate 3-4 translucent shapes from every path you draw by hand like I do nowadays
2. adding new objects to a complex file starts getting super slow (somewhere around 4-5000 paths, less if you're generating lots of virtual paths via various effects) - I'm not sure if this is due to running out of physical memory, or trying to insert new items. in the middle of a very large and complex list of items, or what
3. a few large bitmap effects at 300dpi can very quickly bring IllustratorI to its knees, I'm not sure if this is due to using up tons of memory, unoptimized image convolution routines, or simply having to grovel through a lot of data.
4. also you can do some really terrible things to Illustrator's performance very quickly by applying a distortion mesh to a shape with a pattern fill that contains a lot of copies of its pattern
5. in general there's a lot of ways to send performance over a cliff by making the program generate a hell of a lot of shapes from simple rules - scatter brushes deposit a lot of copies of a shape along the path you draw, art brushes distort a shape along your drawn path, you can generate multiple paths with various programmatic effects applied to them from a single path...
You could probably edit moderately complex files with a theoretical vector package working under the handicap of being interpreted JS running in a neutered web browser. I did nice stuff with Illustrator way back in 2000, on machines that ran slower than a modern box would after the additional overhead of running compiled JS in a neutered web browser. But I sure wouldn't consider it for high-end work.
For that matter, I'm a pretty big fan of the chromebook model, so all the more happy to see web based apps working, even if google seems to be taking a couple steps back in that space. I'm simply unsure if the time/effort it would take, combined with the differing skills combined would yeild something better... more portable maybe, but not necessarily better.
Who knows though.