[stephenr]

As a little experiment of your question (using the original figure actually) I repeated a dozen or so ~lines~ paragraphs of Lipsum... a lot, until I ended up with a text file with a little over 17K lines... 532K words... and either 3.1M or 3.6M characters, depending whether you count spaces.

MacDown (a native MarkDown editor for macOS) opens it in a snap, has no lag editing, and even the rendered preview (which scrolls with the editor side) has no lag. Memory usage was about 150MB.

For shits and giggles I then pasted that content into the OP's linked tool.. The Safari process for that tab is still sitting at 100% CPU a minute or so later, and is unresponsive.

Is this example a bit ridiculous? Sure. But it demonstrates the difference between a native app - oh, and the process for that tab just disappeared because Safari killed it - and trying to make an app in a web browser.

[mjcohen]

I really like MacDown but it has this weird bug of sometimes hanging for up to 30 seconds with a spinning beach ball and then continuing as if nothing happened.