Why read row is so slower than read column.

[yhqruc]

Hi, all:   In my cf, each row has two column, one column is the 
timestamp(64bit), another column is data which may be 500k about.
   I read row, the qps is about 30.   I read that data column, the qps is about 
500.
   Why read performance is so slow where add a so small column in read??
Thanks.