There’s two things going on here:
- delay while email jobs are enqueued
- processing time for sending the actual email
For the first, I’m not 100% confident on this but I think lowering email_time_window_mins means the notifications get queued sooner.
Once the email jobs are scheduled, your sidekiq workers are working through them one at a time. Bumping up the sidekiq workers (set DISCOURSE_SIDEKIQ_WORKERS up from 5 to 10, 15, or 20 depending on server capacity) means more jobs get processed at the same time, so the queue gets emptied 2x/3x/4x faster.