Why would you build a text editor on top of a web browser engine? Who would think that is a good idea?. Of course it is not going to be performant, it is being used for something it was not designed for. On top of that, cross-platform apps built using the one stack are horrible. They never feel exactly like native apps.
If this was 20 years ago, atom would be built with java using swing. It would be just as slow and just as un-native. Java apps on the desktop are ridiculed for a reason. The only difference is that now they are using 'trendy' things.
I think blind Freddy could tell you that, yes you can build a text editor on top of a web browser, but performance, and usability will suck. That is why everyone is so cynical, because it is so obvious.