Assuming "unreviewed" means CR0 and not only "NEW" (which can mean CR-2, CR-1, CR+1 in contrast to ABANDONED and MERGED), this is currently blocked on https://github.com/chaoss/grimoirelab/issues/96

We have this data now on https://wikimedia.biterg.io under "Gerrit > Efficiency" in the "Repositories" widget. It shows "Median Time Open (Days)" per repository and you can sort by that by clicking the column header. Note that you must first set the time span in the upper right corner to something like "Relative: 10 years ago" to have it cover all open changesets in Gerrit. (The "Median Time Open" value will always be lower than the time span set.)

However that Median Time Open value ignores the CR status or Merge Conflicts, so even a CR-2'ed changeset is included in the "Median Time Open (Days)" value. This would require https://github.com/chaoss/grimoirelab/issues/96 to get fixed.

Obviously this does not take into account the number of open patchsets, so if there is a repo with only one open patchset 5 years old and all other patchsets merged, while another repo has all 3-5 year old patchsets open and zero patchsets merged, the latter looks better.

Wondering if on "Gerrit Timing", "Avg. Time Open (Days)" should not only be offered for Submitters but also Repositories. It's a bit of duplication but also more consistent.