Takes time to develope the muscles that make one fast. Have her keep riding and be patient. Time is all she needs. My wife started out on hyrid comfort types. She started improving bigtime after about a year of consistent riding. She's on her 3rd roadie now. She's done 18.1 average for 42 miles. That was pushing it but at one time her best was, 13, then 14, then 15, then 16, you get the picture.
Biggest thing is don't let her get discouraged. Your best bet is to pace with her to keep from de-motivating her. I rode with my wife while gently increasing the effort for her. BUT, I never leave her on rides. Especially if it's a forum ride or similar. If she fades, I hang back with her. Nobdoy means more than making her feel like less of a rider. Maybe that's why she's been out ther with me for 10 years!
Give her time! But yes, you can improve bigtime on a comfort bike. Couple of hints. Make sure she has a good comfy saddle and good tires. Don't try to sneak by with giving her crappy spongy soft tires then expect her to keep up. Seen that too many times!
For the record, my wife's bike is 3X's the price of mine. Her tires are only double the price though!
...Pays off in the long run!