Update command too slow

Hello,

We have a table cm_quotastates which has exactly
4624564 rows and 25 columns and 9 indexes... Out of
these, our code retrieves 75262 rows and modifies just
one column in each row... but updating these to
database is taking some significant time (around 20
minutes)... Tried the following with the update

(a) Tried updating after removing all the 9 indexes
associated with the table
(b) Tried updating the 75K rows in batches
(c) vacuum analyze the table before updation

but none are helping and update still takes the same
amount of time. Is there anything else that can be
done so that update takes lesser time... Also, where
can I find info about how postgres update actually
works?

Thanks,
Venkatesh

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