I can see why created_at is needed there, its the query that is trying to find new topics it takes into account a “start date”.
Added the index and can confirm perf is way better:
https://github.com/discourse/discourse/commit/20bee937b70e04aa5afb09635b8aa85866dee588
Thanks for your help here @DeanMarkTaylor !!! closing this for now I feel it has been fully addressed 