Importing Data into SharePoint 2010 from SQL Server

I'm creating a PowerShell script to migrate data from a SQL Server database into SharePoint 2010 lists. I have to read 500,000+ records from the database, perform some operations on it and then import it into various SharePoint lists. I can get this to work but essentially I'm looking for the fastest way to import this amount of data into SharePoint.

I have tried using the SharePoint API to add and update list items directly but I left this to run over night and it wasn't finished in the morning.

I have also tried using the lists.asmx web service to upload pre-generated XML for each item however this approach doesn't seem especially fast either. The XML for this is broken down into batches to avoid timing out the web service request. I tried uploading the batches usin开发者_StackOverflow中文版g asynchronous web service requests but that basically killed the SharePoint site so now I'm just using synchronous requests which is also slow.

Any suggestions for how to read from SQL Server, transform the data and add it to SharePoint lists?

Addition

When the items from the database are manipulated to be inserted into the SharePoint lists, this results in 771,900 individual list items to be inserted.

Disabling workflows on lists can greatly improve performance. New estimated time for import is down to just over 7 hours from the original estimated 29 hours.