Hi.
First, I don't think it's all that useful to think about "speeding up electrons" as the method of heating food.
Microwaves cause water molecules in food to vibrate, producing heat that cooks the food.
[https://www.fda.gov/radiation-emitting-products/resources-you-radiation-emitting-products/microwave-oven-radiation]
So, straight away, "slowing down electrons" won't really be a way to cool food down. What you'd really want to do is slow down the movement of whole atom-sized and molecule-sized particles.
Next, do you really want to use magnetic fields as your tool for the cooling, or are we free to suggest other things?
Best Wishes.