My understanding is, that the rasterization process also happens per tile. One of the cases where this can cause performance problems is, when you want to read

My understanding is, that the rasterization process also happens per tile. One of the cases where this can cause performance problems is, when you want to read the output of a previous render pass. If you want to be able to read arbitrary parts of the output of the previous render pass, the output buffer probably needs to be copied from the tile memory into a memory that can hold the whole buffer at once. Furthermore, this also means that all tiles of the previous render pass need to execute before the next one can run. This limits how much work can be done in parallel.