Lag When Editing Large Notes

### Expected

Editing a large note should work smoothly with no performance issues.

### Observed

A large note (200-300 lines of text, 50kb or more in size) makes the editor lag significantly

### Reproduced
> 1. Create a new note
> 2. Add 200-300 lines of text, 50kb or more
> 3. The editor lag—inputting text, scrolling up and down, selecting text

Apple | Macbook Pro 15 in 2018 | 10.14.4 | 1.5.0

Context:

#2180764-zen

Start by reproducing the lag in a new note containing 200–300 lines or at least 50 KB on the reported macOS setup. Compare typing, scrolling, and text selection with a small note, then trace the editor entry point responsible for those interactions. Done means large notes support smooth editing, scrolling, and selection without regressions.

Written by the indexing model from the issue text.