When the narrator is enabled and chat suggestions are being shown within a command block, holding the up/down arrow keys to quickly change between suggestions will cause the game to freeze until the arrow keys are released.

Reproduction steps

1. Enable the Narrator with ⌘+B.
2. Place and open an empty command block.
3. Hold down ↓.
4. Observe that the game starts to lag.
5. Release ↓.
6. Observe that the game stops lagging.
7. Repeat the process again with Narrator disabled.
8. Observe that the game does not lag.