It depends how many requests (both frontend and backend) each user triggers and the odds of a big latency spike on at least one of them. For single requests, t

It depends how many requests (both frontend and backend) each user triggers and the odds of a big latency spike on at least one of them. For single requests, targeting 95% would probably be just fine back in the days of static sites served by a single Apache instance, but that doesn't describe many modern systems. Nowadays, a single HTTP request to a load balancer can trigger a flurry of blocking queries to fulfil it. (eg a distributed ElasticSearch query that can't combine the results from each node and return until EVERY node has responded) then your worst-case performance will quickly begin to dominate.