The honest answer -- Microsoft has a lot of people with experience in building these things, so they aren't making the typical mistakes that Atom made with their architecture. For example: Atom was rendering dom nodes for your whole file -- not just the parts that were visible. Using the DOM as a stateful container of all that could be visible does not scale really well.

Compare to something like the ACE editor (an OSS editing component that you can plug into any website) which is lightning fast -- it only renders what you see though -- but the filesize is not at all relevant to the performance.

These are all techniques that people who ever wrote complex UI toolkits are incredible aware of. I don't think that describes anybody who worked on ATOM ever, which means they just have to relearn and re invent this stuff until they get it right.

Generally, these days a lot of developers get a lot of experience combining existing technologies. Proper engineering skills, or even being aware of the data structures you are using and their algorithmic complexity, how cache friendly it is -- all of this is prioritized much less now.

So Atom answers a simply question. Has hardware become significantly fast to not care about proper engineering? No, not really and it never will. If your datastructure is structured in a such a way that inserting a line all the way at the top requires a relayout of every line below that, for example, you will run into real limits.