Hi Eduardo.
The PARTNER performance issue is, unfortunately, known. I had the same issue and I tried all the steps you did. Disabling the integration with CRM (BC-MID) brought 20% improvement, on my server, but that still wasnt enough.
What I did, and what I can suggest you to try, is to make a distributed load with small amounts of data. Make that 500 or 1000 business partners per job. Make sure (with your BC team) that you have enough available background job slots. You could try with 10 to 20 parallel jobs. For additional improvement, you could try disabling writting statistics and changing the commit interval.
Hope this helps.
Marko