Large files lagging editors is usually because the entire file needs to be parsed for the editor to know how the syntax highlighting and error reporting should

Large files lagging editors is usually because the entire file needs to be parsed for the editor to know how the syntax highlighting and error reporting should work. Making that random big_file and opening it and editing it should be trivial under any editor (works perfectly fine under Emacs because it gets opened in fundamental mode, even opening it in hexl (hex editor) mode is fine), opening a 40M JSON file is a different can of worms (because it's a larger file Emacs will offer to open it in fundamental mode anyway, which means sacrificing QOL features for performance).