According to the answers above from Discourse team they do not see any issues with Discourse. I resolve this issue in a dirty way with direct hacking of database. Here is my solution
cd /var/discourse
./launcher enter app
rails c
Topic.exec_sql("UPDATE users SET last_emailed_at = now() + interval '30' second * id WHERE last_emailed_at > ('2020-03-14'::date) AND last_emailed_at < ('2020-03-17'::date)")
Please update last_emailed_at range in SQL query. I had 4K last_emailed_at in 1 day on 16 of March. So, now all users were optimized to 3 days with interval 30 seconds.