The best way is to ask your husband to tone it down a notch when you're riding together. I'm not trying to be snarky, because I've been there and done that and realized that riding with my wife is not the time to try for a workout. Going easy anyway, it doesn't matter how easy so there's no issue from my perspective.
Changing things on the bike isn't going to get you to 18 mph from 10-14. A lot of things will help some, but in the end he's likely going to have to compromise with you. You'll both get stronger, you're not really holding him back.