There is a long standing issue that happens when there are less tiles than available cores situation at the end of every pass. In case of an scene rendered with several passes, this downfall will happen at the end of every pass. This poses a lesser problem with low sampling, but with high sampling settings those last tiles take too much to render while most of the computing capabilities stay idle. I think most of YafaRay users are rendering with 4 cores if not 8.
With the limited knowledge I have about maths, I think a good solution could be that we render a sucession of increasingly smaller areas of tiles of the same size, following a mathematical pattern for consistency and formal beauty, like a Fibonacci sequence, golden ratio or just geometric series, fractals.