I gave blood last Friday as it happens and was up off the bed almost before I'd lain down. It took quite some time for some of the others but not me.
Eat some fish and shellfish, if you think you're blood's "sluggish". You should be eating them anyway:
Why should we eat more seafood to keep our brains healthy? · Food & nutrition · Institute for Food, Brain and Behaviour
As for McDougall, I suggest he takes a very large dose of warfarin, and if it does something more permanent than thin his blood at least it will give us all a rest ...