Reproduction steps:

1. Create a new Unity project.
2. Create terrain.
3. Select it.
4. Select Raise/Lover terrain tool.
5. Move mouse over terrain.
- Notice how smoothly brush follows the mouse.
6. Move mouse out of the Editor screen.
7. Move mouse over terrain.
8. Brush updates with long refresh time(GIF attached).

Note:
- Reproduced with a single display.
- To refresh/workaround this issue, select any brush type.
- iMac specs attached.
- May be related - case: 768810

Reproduced with:
5.4.3p2, 5.5.0f3, 5.6.0a5

Resolution:
View update is slower when not focused.