Is it a myth that when you quit smoking your body metabolism slows down?

I quit smoking in March of 2018. By June of 2018 I had gained 20 pounds. It’s been a fight, but I’ve lost at least 5 of those pounds. I’m down to taking in maybe 1000 calories a day. It seems like I should be losing more and much more quickly. Now I’m trying to remember what it was like when I quit smoking the first time, in the early 80s, and stayed quit for 8 years. It seems like the same kind of thing happened, but my body eventually got back to normal…but then I started having babies so whatever.

I looked it up and internet sources suggest smoking increases your body metabolism, and quitting slows it down. I don’t know whether to believe it or not.

If it’s true, when will I get back to normal?