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Raspberry Pi is 4.5 times faster than the Cray 1 was in 1978
(www.roylongbottom.org.uk)
This is a most excellent place for technology news and articles.
Its strength was in running the same operation on large sets of data rather than general purpose computing. So specialist hardware would need to be developed for real time input and a graphical display (which would need to be able to draw the screen from the data the Cray produced. )
I think a better comparison would be a modern GPU.
A Cray 1 could do approx 160,000,000 floating point operations per second. A modern GPU can do 1,600,000,000,000 per second.
If I'm reading that right, it's 2 orders of magnitude greater? (Math is not my strong suit)