Why are shift registers are so slow

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I am dealing with large amounts of data, arrays in excess of 65000x15.
I need to build the array in a loop.  When I use the shift register
approach it takes forever.  The fastest way I have found so far is to
write the rows out to a spreadsheet file and read them back in.  This
is fairly quick, but seems rediculous to have to do.  Memory should be
much faster then a HD