The Dehli castle upgrades (Boiling Oil, Slow-Burning Defenses, and Village Fortresses) take longer to research when you add more scholars to the network. This should obviously have the complete opposite effect. Village Fortresses starts at 5 minutes, but becomes over 14 minutes long when adding 9 scholars into 3 mosques.
Thank you @Iamdreadx! We are aware and working on it.