Would it be helpful to establish some clear examples of what "too fast" means? i.e., what behaviours to avoid and what signs would mean she's ready to go further? My reason for asking is that speed is relative, and too fast for one person is too slow for another.
I generally interpret "go slow" as "I want to take this far / all the way, but not quite yet". But for some people, "go slow" is code for "never go past this point" and it's important to know the difference, and what to look out for, what to avoid, "how to go slow".
Especially since this relationship started in swinging. In many relationships, "too fast" refers to physical intimacy as well as emotional. But if you've all had sex already, then presumably "too fast" refers to emotional intimacy, and the expression thereof.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker