Note with many ★ characters has slow typing

### Expected
Typing in a note should show instant feedback, independent of the type of characters in the note

### Observed
A large note with ~1300 ★ characters shows some noticeable slowness when editing the note in Simplenote macOS. If you replace the ★ with ☆, the slowness disappears. (possibly something unique about how this character is represented in macOS code? https://www.fileformat.info/info/unicode/char/2605/index.htm)

### Reproduced

1. Create a note with many ★ characters
2. Start typing rapidly
3. Notice the cursor is falling behind the characters that you are typing
4. Notice that if you replace the black stars with ☆ characters, the slowness is solved.

See this folder for videos and sample text showing the issue: https://drive.google.com/drive/folders/17de2xj-bMORWlYXjwzIVWOZF3Kqg0kRn?usp=sharing

Note the slow typing note seemed fine on iOS, Android, and in the electron app tested on Mac and on the Web.

Tested on:
MacBook Pro running macOS Mojave 10.14.6 (18G103).
Simplenote macOS version 1.7.0

Start by reproducing the issue in Simplenote macOS with a note containing about 1,300 ★ characters, then compare typing with ☆ as described. Profile rapid typing and text editing to identify the source of the cursor lag. Done means typing provides immediate feedback for the large ★ note without regressing behavior for other platforms or characters.

Written by the indexing model from the issue text.