How to efficiently convert IList<byte> to byte[]?

I have problem with efficiently converting an IList<byte> to byte[]. I'm working with Xamarin Studio with OpenCV to process images and Fotoapparat to take pictures. I want to convert the list to an array because OpenCV expects an array not a list. I know the 'easy' way to convert with ToArray(), but it takes too much time (more than 8-9 sec to convert). For reference, my IList<byte> has a length of 5564448.

Does anybody know how to efficiently convert IList<byte> to byte[]?