..ok.. It does already use an optimisation here. If you draw a big rectangle, it'll use at most 8 triangles to draw it.
There are a few exceptions: Perhaps you're rendering into a target where the clip-region is not a simple rectangle? Or have you rotated or skewed the context?
The place to look at what's going on is juce_RenderingHelpers.h - when your fillRect call goes in there, it'll do different things depending on the type of clipping involved. For simple clip rects, it should use the code in RectangleListRegion::fillRectWithColour, which is optimised to use a minimal number of triangles. If it ends up in one of the more complex clip classes, then it may have to use a line-by-line rendering method.