How to use repository pattern and unit of work for bulk insert with EF and give more performance?

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C#

EF6.0

Hi. I am working on a project that use unit of work and repository pattern with orm EF6.
i want insert a list of object about 800k record into database.
i send every 15k or 10k record to instance of repository and then commit it.
it is too slow. how can i give more performance?
i dont want to instal EF.Extensions.
my problem is about foreach loop. its too slow add to repository.

What I have tried:

foreach (var imei in command.IMEIItems) {

var newIMEI = new IMEI(imei.IMEINumber, command.PromotionId);
_imeiRepository.Add(newIMEI);
}

_unitOfWork.Commit();