It's going to be a mix of:
1. JS doesn't support multithreading, nor many other features that are useful for performance (e.g. mmap). This severely limits what you can do and makes it hard to scale up by parallelizing.
2. JS is a very pointer heavy language that was never designed for performance, so the CPU finds it harder to execute than old-school C++ of the type you'd find in Word. It's hard to design tight data structures of the kind you'd find at the core of Word.
3. The browser's one-size-fits-all security model sacrifices a lot of performance for what is essentially a mix of moral, legal and philosophical reasons. The sandbox is high overhead, but Docs is made by the same company as Chrome so they know it isn't malicious. They could just run the whole thing outside of the sandbox and win some perf back. But they never will, because giving themselves that kind of leg up would be an antitrust violation, and they don't want to get into the game of paying big review teams to hand out special Officially Legit™ cards in the same way that desktop vendors are willing to do.
4. The DOM is a highly generic, page oriented structure, that isn't well suited for app-like UIs. As a concrete example Chrome's rendering pipeline contains an absolute ton of complexity to try and handle very long static pages, like tiled rendering, but if the page takes over rendering itself like Docs does then all this just gets in the way and slows things down. But you can't opt out (see point 3).