I'm aware that there are some commercial libraries and that there's AddRange, but AFAIK AddRange does piecemeal INSERTs under the hood.
I'm looking for a free utility that I can use to add a collection of new entities all at the same time - does one exist for EF6?
I would suggest you take a look at N.EntityFramework.Extension. It is a basic bulk extension framework for EF 6 that is available on Nuget and the source code is available on Github under MIT license.
Install-Package N.EntityFramework.Extensions
https://www.nuget.org/packages/N.EntityFramework.Extensions
Once you install it you can simply use BulkInsert() method directly on the DbContext instance. It support BulkDelete, BulkInsert, BulkMerge and more.
BulkDelete()
var dbcontext = new MyDbContext();
var orders = dbcontext.Orders.Where(o => o.TotalPrice < 5.35M);
dbcontext.BulkDelete(orders);
BulkInsert()
var dbcontext = new MyDbContext();
var orders = new List<Order>();
for(int i=0; i<10000; i++)
{
orders.Add(new Order { OrderDate = DateTime.UtcNow, TotalPrice = 2.99 });
}
dbcontext.BulkInsert(orders);