Real time collaboration is an awesome feature and essentially what justifies Google Docs' existence, as it's behind Word in practically every other area (though I find Sheets more intuitive than Excel, that might just be familiarity).
The technology to do RTC is not particularly resource intensive on the client side. Nor is it web specific: the native Android versions of Google Docs don't use the web but they do support RTC.
RTC is enabled by an algorithm called "operational transform". It's a very clever algorithm that is rather tricky to implement properly, but it doesn't involve loading huge datasets or solving vast numbers of equations. It's ultimately still just about manipulating text. You could have implemented the client side part of it on Windows 95 without trouble, I'd think. At least I can't see any obvious problems with doing so, assuming a decent Windows 95 machine like one with 8 or 16mb of RAM.
OT does, however, require the entire app to be built around the concept. You can't easily retrofit it to an existing editor.
The reason Word 95 didn't have Docs style realtime editing is simply because back then networks were kind of rare, slow, crappy and word processor designers didn't know about the OT algorithm back then because it was still being researched by academia.
The real question is - if we had a better client side platform on laptops and desktops, one that supported some of the best features of the web without the rest, would Docs RTC still be possible? Surely yes!
