Peralta is a Derek Jeter type defender. He makes very few errors but has average to below average range.
I think you are underselling what Peralta did defensively if you compare him to Jeter. But then, maybe I'm under the influence of Dave Cameron. I love Iglesias's defense, but that was maybe the position in their infield that was hurting them the least with the glove last year. You could say that Peralta is better defensively than Desmond. No, my knock was focused on the corners, and I'll probably take Espinosa at 2d over Infante (or whoeve they were troting out last year), too.
The BABIP of lefties against Fister so far this year is close to his BABIP last year. This year, it is almost .100 less against righties. Some of that I would not bet on staying, but some of that is Rendon v. Cabrera, I would think.
Another weird thing about Fister this year is, so far, he's getting more fly balls than ground balls v. lefties, but he has also chopped his HR/9 to lefties. I think some of the skepticism you see about him going forward is an expectation that those flies will start to carry a bit, while the BABIP will come back to some towards .300 (career .297 or so), and that his strand rate will dip below 80%. I think some of the suppressed BABIP is real, due to better infield defense, and that he is probably one of those heavy sinker types that can maintain a low HR rate. He's a really nice fit for the Nats.