Huh, that’s weird, because the Twine app doesn’t DO anything with the passages AFAIK? It just edits them as text, so it doesn’t care how complex your logic is, and I can think of multiple games that have 1500-2000 passages and… they’re a little laggy to scroll around, but not bad. I know it was a lot worse at some point in the past, but I thought it had been pretty decent for a decade or so now.
And I don’t usually own particularly powerful computers…
The big issue is drawing the SVG layer that represents the arrows. Hiding it improves performance immediately. That’s why I said that I felt it was connections that mattered more than passages.
Passages do matter as well. A large enough number of passages can cause the passage map to scroll badly, and fail to draw the background image consistently.