An excellent decision, I guess. I'm somehow thinking way too much about such details instead of starting any project. In my previous job, we have used PHP at the backend, but somehow, I like the syntax of python much more. And the community and number of projects around Django are just mind-boggling. So you are probably using Django-REST for the API?

With react-virtualized, you are already on the right path as virtual rendering - rendering only the currently visible cells - is the key in regards to the performance of DOM-based approaches. But if you want to reach google sheets performance, there is probably no way around canvas-based rendering (e.g. https://github.com/fin-hypergrid/core or https://github.com/TonyGermaneri/canvas-datagrid). Office 365 seems to use a canvas-based approach as well. The problem with the canvas-based approach is that interaction like mouse clicks and key navigation is a lot more complicated. I think one of the fastest DOM-based grids out there is https://github.com/6pac/SlickGrid. But there shouldn't be too much difference in performance to react-virtualized as both are DOM-based. Often these components are called "grid" if you search for alternatives. Feel free to contact me if you have some specific questions :-)