Select query takes long to execute

Hello,
I'm running a simple query on a table and I'm getting a very long
response time. The table has 56,000 rows in it. It has a full text field,
but it is not being referenced in this query. The query I'm running is

select row_key, column1, column2, column3, column4, column5 from table1
where column6 = 1 order by column3 desc limit 21;

There is an index on the table

message_index btree (column6, column3, column7)

Column 3 is a date type, column 6 is an integer and column 7 is unused in
this query.

The total query time is 6 seconds, but I can bring that down to 4.5 if I
append "offset 0" to the end of the query. By checking query using "explain
analyze" it shows that it is using the index.

If anyone has any ideas as to why the query is taking so long and what I can
do to make it more efficient I would love to know.

Thanks
Kevin