The site is served via AWS CloudFront. It is SPA app hosted on S3. The API is on Heroku. No mem-cache anywhere. To search we use ElasticSearch hosted on AWS. We bump up the specs to speed it up but that was not the core problem.

I suspect Heroku Postgres DB. We didn't used some Hobby version and I think it is the real bottle neck + some unnecessary middleware for the API that also requires DB.