Metabolism only slows slightly unless you do a severe restriction for a very long time. I averaged under 1000 calories a day for over 2 months. My rate of loss was about a half a pound a day all the way through; there was no significant change in my metabolism. That was absolutely not healthy and I am not recommending it, just pointing out that short term stints of severe caloric restriction don't destroy your metabolism. When they put severely obese patients on VLC diets under a doctor's care, they go months without a break and continue to lose at a high rate.
When I was losing at the high rate, I still had some periods where my weight didn't change for a few days and then would plunge. Sooner or later the math caught up with me.