Long? If you look at http://www.ams.org/notices/201410/rnoti-p1268.pdf (google “AMS notices backlog” for other years), that is fast for a paper in a mathematical journal.

Yes, this is not a paper, but they may have more letters queued up, and this may actually be surprisingly fast. That June/July issue might have gone to print in early May.

In the end, this is still a print journal, with its limits on page counts, expectancies of ratios between articles, reviews, and letters, etc.