You may notice a difference in elapsed time between PAM Http Get operator and HttpRequester (Firefox Add-on). See attachment for screendumps of each.
It is expected that PAM will take more time than an external tool for the simple reason that PAM is doing more. For instance, because PAM has to persist its state in case there is a crash and has to recover, so PAM will be serializing the results into a database, and that is just one example of additional work that PAM has to perform as part of its operation when processing the request. The timing differences you are seeing are consistent with what we expect to see in the the current design.