Is it possible to improve the execution performance of a query with 'simple' joins?
Many outer joins are not a best option when dealing with big tables. In a nutshell, your plan might not be as optimal as it could be when using outer joins.

Wrong RENDA_FIXA table estimation is the reason why you are getting Parallelism and Hash joins everywhere even though you are not expecting many rows in the first place.

You can try OPTION (MAXDOP 1) hint to see if that any helps.
Also you should check statistics on RENDA_FIXA and GRUPOS_USER_2804 tables. 

The reason why you are getting that plan is because it expects 120,000 records, instead you're getting 1,800 that's why left loop joins helped you.

My advice is to try to deal with RENDA_FIXA table first, get the correct estimations out first, so the query optimizer could try to create a right plan.