Insert overwrite for 2 GB data

I am running
insert overwrite table emp_rc partition(year) select *,year_num from table1;

table1 size is 1.8 GB and this table is textfile, while emp_rc is RCFile.
When i run this sql, it takes 1 hour, I have set mapreduce.job.reduces = 30 and now it takes 15 mins.

Can you advise, what else I can do to improve performance.
Can you explain how to analyse explain plan for such type of HQL.