Prius B mode during engine break-in?

Slower than which?

Slower than possible with foot braking, yes. Slower than "D" mode synthetic engine drag, no, "B" mode charges at a faster rate. But it still ends up with less total charge gain because a portion of the available braking energy is wasted by throwing it into spinning the engine. The faster rate is more than offset by the shorter time.

More than the brake pedal? If so, then you are braking too hard and short for effective braking regeneration. Done well for efficiency, from above mid-20-ish MPH, the foot brake should always harvest more charge than can B-mode. From below that speed, B-mode is capable of squeezing out a tie, but not a win.

As Jimbo points out, your zip code is nowhere near anyplace where B is useful, at least at this time. If you wait until the peak of the next Ice Age, if one ever comes again, the shoreline could fall as much as 600 feet. Maybe then B mode could possibly become just marginally useful, if a new road is built down the newly exposed continental shelf.