Reproduction steps:
1. Open attached project
2. Open "TestScene" scene
3. Open test.cs and uncomment "areaOfSliceInTextureToDraw = new Rect(0,0,33,33);"
4. Save test.cs
5. Play/Run "TestScene" scene (Notice how much time it took to process)
6. Open test.cs and uncomment "areaOfSliceInTextureToDraw = new Rect(0,0,31,31);"
7. Save test.cs
8. Play/Run "TestScene" scene (Notice how much time it took to process)

Note: For a 31x31 rec it will take 0 ms per Sprite to process and 0 ms for 25 Sprites. For 33x33/32x32 it will take 14.48 ms per Sprite to process and 362 ms for 25 Sprites. And for other rects under 31x31 it will take 0 ms.