If it’s actually a slow period at the market then I’d have no problem with it. If she was ignoring customers who are trying to get her attention (not just browsing) then it’s a different matter.

First I’d ask: Is he accurate? If he picks up the pace will he be less accurate? And which is more important, pace or accuracy?

I would probably show him his stats and say it’s pretty slow compared to others but since he’s new it’s not a huge deal, and then I would challenge him to pick up the pace by hitting a (very slightly) higher target. I would tell him you appreciate his effort and his cultural fit and you want to make it work. And I would review again after another month or so, and if he’s picked up the pace I would do the whole thing again but set a higher target.