Good question!
Without the speed change:
- Clear screen: 588 ms
- Draw 96 characters of text: 347 ms
- Frame a rectangle: 278 ms
- Fill a rectangle: 4687 ms
With the suggested change:
- Clear screen: 229 ms
- Draw 96 characters of text: 148 ms
- Frame a rectangle: 120 ms
- Fill a rectangle: 1753 ms
With the suggested change and write-through caching enabled:
- Clear screen: 230 ms
- Draw 96 characters of text: 147 ms
- Frame a rectangle: 78 ms
- Fill a rectangle: 976 ms
The activities which don't involve reading back were within 1 ms of before, but the ones which involved pixel drawing were somewhat faster.
Personally I think that writing all 96 characters (over half a 8x21 line screen) in just over 1/7 of a second is pretty reasonable.