15min is clearly more reasonable than 30min so humans can compete better with bots. It was discussed and many were of your opinion but it was easier to just change a parameter and has less risk of unintended consequences.

Even if it was to take 30min to read an article it usually doesn't take more than 15min to get a good feel of the value of a post.

Also given 10 votes a day as optimal voting frequency it's less likely anyone would actually spend 5 hours a day (30min*10) reviewing content than 2.5h.