CRM ON-Premise External Database Integration

I have a requirement to process 10's millions of records on a regular basis and would like to use an external SQL Server database to do the heavy lifting and then take result sets back into CRM. The issue I have is the speed of transferring the data. We have tried using Powershell but that is too slow Next we are profiling TPL to do the same task and then lastly using Scribe. Has anyone developed a service that has a similar pattern and if so what woudl they suggest as a good way of moving data between CRM and SQL Server say a million rows a day.

Thnaks for any help

Mike