Just to confirm:
I dumped all of my notes from my insanely large apple notes (about 16000 lines of text) and pasted them into Google Keep, Notion, Google Docs. With the exception of Google Docs the rest of them flat out froze and I had to kill my browser. Stop trying to tell us that the browser is the answer to everything when most web apps cant do the job of Notepad.exe or vi
Tried putting 20k lines into it. Loaded instantly, allowed me to scroll and edit flawlessly.
But I get your point. I'm on a pretty decent 2022 iPhone, and I'm sure at some stage I would run into a performance hit. But not at 20k lines.
So, one out of three webapps that you tested could handle this much text. It suggests that the problem for the other two is their implementation, rather than any limitation of the browser.
Of the two that failed, did you also try the app versions to see if they failed too? I really doubt the Notion app could handle 16000 lines of text.
You cant slap a plain text field and assume that emulates the actual experience in any way.