Bulk copy is about the fastest you could do in C# or PowerShell, but you would still be moving a ton of data back and forth over the wire between the DB server

Bulk copy is about the fastest you could do in C# or PowerShell, but you would still be moving a ton of data back and forth over the wire between the DB server and the "jump" server running the script. I would have tried a hybrid approach with the dynamic SQL one. Dynamic SQL sprocs like that don't seem to generate good query plans (at least not with a lot of prodding) in my experience, but if the dynamically generated SQL was done in the C# or PowerShell side rather than a sproc I feel the query analyzer would be less confused about individual queries coming out of the scripts. The only other advice experience from a past life I have to offer is that I often saw better performance from T-SQL MERGE than T-SQL INSERT FROM. Dynamically generating MERGE statements from C# is not fun, but in that past experience it would give the best results for single-server data migrations like this. (Multi-server there's not much you can do beyond Bulk Copy.)