When I add up a column in the select the performance fall down

Hi guys. It's a long that I'm struggling with the performance and I'm checking as more as I can. I thought I had addressed the issue but now...I got a table with 225,000,000 rows and 22 column. Now, if I run the select below

Select customer, contract from bigtable group by customer, contract

it takes four/five minute (4.23). Fine. But if I run the query below

Select customer, contract, employer from bigtable group by  customer, contract, employer

it's taking, so far by the moment is still running, more than 50 minutes when it used to take 10/15 minutes...

Any suggestion?