Girlfriend takes a long time to cum, even with vibrator

I’ve (male) had a couple relationships with girls before my gf, and I could make them cum fairly easily and I never had a problem with it, I was especially good at oral sex. However, it is my current girlfriend’s first relationship and first time having sex. We’ve been having sex for a month or so now and it’s really good for both of us, I do plenty of foreplay, give her oral for a long time, we have penetration sex etc. I bought her a vibrator that we use during sex and the only thing that can make her cum is the vibrator. She says sex feels really good but it can’t make her cum, which is fairly normal, my previous girlfriends had this too, so I would always make sure to make them cum from oral, I didn’t need a vibrator. She says it feels like there’s pressure when I’m watching and it makes it difficult to cum. Ofc I always make sure she cums, but we have to use the vibrator (it’s a lovehoney bullet) in between rounds and it takes her a long time. Is this normal, what can I do to help her cum without feeling pressure, any toys that people with a similar problem recommend?