While upgrading pandas from 0.7.2 to 0.9.1 we have bumped into slowness of certain groupby().sum() operations. Here is a simple example:
N=10000
aa=DataFrame({'ii':range(N),'bb':[True for x in range(N)]})
timeit aa.sum() # fast
timeit aa.groupby('bb').sum() #fast
timeit aa.groupby('ii').sum() # very slow (~ 1000 times slower)
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