Crystal reports, local service account for app pool and performance

A small background about the issue, I have a web-service that was creating some reports in the pdf format and putting it at some location. It was taking about 15 seconds for 8 reports on development server but on qa it was taking around 80 seconds. While investigating the issue, I found out that it was running under apppoolidentity account on windows server 2008. When I changed it to local service, the time dropped from 80 seconds to 15 seconds. While that fixed the issue, I couldn't figure out why this happened. Just for experimenting, I changed it to local system and network service as well but that bumped the time up to 80 seconds. I know that local service has more privileges than the apppoolidentity account but how can I describe this difference? Why does crystal recommends that LOCAL SERVICE permission must be setup on your application pool ?