BUG: Speed degradation with categoricals
I have noticed some major speed degradation when using categorical data types.  For example, a data frame with 1 million rows that sums 10 float columns and groups by two columns (one a date column and one a category column).  The cardinality of the category seems to have a major effect.  When grouping on category column of cardinality 10, performance is decent (query runs in 150 ms).  But with cardinality of 100, the query runs in 10 seconds.  If I re-generate the data frame without  categorical columns, the same query that took 10 seconds with categoricals now runs in 350 ms.
