Should I finish her sentences for her or let her finish?

Should I finish her sentences for her or let her finish? Hello! I am dating a new girl who is super sweet and kind and I want to be respectful. She has a stutter so she hangs up on words probably ever few sentences and I typically know what she's about to say and I've caught myself jumping in and "talking over her" persay a few times. I'm a very, very fast talker, so finishing her sentences kind of comes naturally to me, but I want to know if this is disrespectful to her. If it is, I'll make a conscious effort to let her finish moving forward; I just don't know the etiquette.