Nice, but in the kind of dev work I do, I need to be able to work with text files that are hundreds of MB, even over a GB, in size, and CotEditor chokes on 200 MB text files...
Why do you have text files that are 200MB? Why don't you separate that into multiple files? 200MB of text? Moby Dick is 1MB in plain text. The Holy Bible is like 4MB in plain text. What is in your files? I think you may be doing something wrong.
We deal with data sets that are several GB in size, and so I'm already using just a subset of the data. These files are loaded by programs into memory and serve customers via a REST API, and it's not uncommon that we have to manually inspect or change the files from time to time.
Our use of large text files isn't only internal; our software also processes delimited text files for customers, and these can run into the same size ranges, and it's out of our control. But when the service bombs, we need to be able to prove to them that their file is malformed, and if it isn't, we have to fix our data or our code. That usually requires inspection of text files.
But that's all beside the point. This looks like a fine source editor for basic needs, but I wouldn't consider it a tool for general "plain-text" editing like it says on the site.
Whenever possible, the machine should accommodate the human rather than vice versa. Viewing a 200MB file in a text editor is not an outrageous request on a modern computer. Maybe he's not working optimally, but why should he change if his approach should work, and does work fine with other tools?
No single file of source code should ever be even close to 200MB, and no data file (CSV, XML, etc) of that size should be manually edited in a text editor.
Vim and Emacs are quite well-suited for textual manipulation of very large data files. Use of a macro is a very pleasant alternative to piping the file through sed/awk/etc. for a once-off transform.
This sed substitution thing seems to be a cute trend to express "fixed that for you". It goes nicely with programmer's propensity to express themselves in needlessly arcane ways.