Nowadays browsers use relatively a lot of resources. I don't know how ChromeOS handles tabs, but I imagine running everything in a browser takes a toll on performance.

I remember the agony I experienced when I needed to use Word365 online for a project. The javascript implementation was so slow and resource heavy that I was almost going to go crazy. Google Docs on the other hand worked fine most of the time. But none of them could replace a native MS Office or Libreoffice. Maybe with webassembly things are going to get different, but adding abstraction layers come at a cost.

Also these machines need to run videos and process pictures and maybe movies. With those requirements performance doesn't hurt.