Built-in(?) "document" editor chokes on medium-sized files

When I create a Text Document (which will get saved as *.md), editing the document gets laggy when there is a lot of text.

E.g. marking a word and making it “bold” will take roughly one or more seconds, which makes it pretty unpractical.

I struggle to find the app name and the github repo, would like to create an issue there. If somebody could shed some light so that I get in contact with the devs, I’d be glad.

Okay, this is the repo:

… and I created this issue