Why not set backlog on Heroku?
First read this: https://mikecoutermarsh.com/adjusting-pumas-backlog-for-heroku/
Now we're on the same page. Heroku will re-route bounced requests from your dynos but it assumes when this happens that your entire app is saturated. https://devcenter.heroku.com/articles/http-routing#dyno-connection-behavior. Each connection gets delayed by 5 seconds, so you're automatically being docked 5 seconds per request.
If you're setting your backlog to a low value (i.e. if you'll ever actually ever hit backlog) then you'll be in for pain. When you get a spike of requests from slashdot/reddit/whatever you're telling your dynos that you would rather they failed then returned slow responses. So this does mean that some requests will be served, but all the others will fail.
If your app ever hits your backlog (no matter what value it is) it is an indicator you don't have enough throughput and you need to scale out to more dynos. If you set this to an arbitrarilly low value, that point comes