Perhaps one way of encouraging new reviewers to get on board and speeding up the process is to offer a priority queue for anyone who completes a review of another theme?

I think that contributors could be rewarded like this and many theme authors would jump at the chance to get their theme reviewed sooner simply by reviewing someone else's, especially since the wait times have gotten long.

It would also encourage them to familiarize themselves with the review process in more detail themselves and so more quickly identify issues in their own submitted themes, reducing the workload on the other reviewers.

Another similar option, which may help resolve the backlog, is to (also?) offer a paid priority submission (that goes to the reviewer on completion) ... I realize this may conflict a little with the "free" philosophy of wordpress, but as a temporary solution it could be tested at least until the backlog is reduced to a manageable size.