The speed largely depends on the implementation and on the features of the processor. A very rough estimate of the expected speed is extrapolating cycles per byte to your specific device.
numbers do vary , but let it be conservative and suppose your SHA-256 implementation takes about 25 cycles / bytes. Taking a response tells you that you get 1/25 bytes per kile. The 200-MHz processor has 200 million cycles per second, so this gives a theoretical throughput of 8 million bytes / s, which is ~ 7.63 MB / s.
But this is just a very crude assumption, if you want accurate numbers, the best thing you can do is just check it all yourself.