Performing load test through SOAPUI tool shows low TPS for invoking ESB services requiring HTTP BASIC Authentication

Issue

• It has been observed that while performing load test through SOAPUI tool shows low TPS [1] results while invoking ESB services (which are internally calling a WebService through SOAPProxy action) which require HTTP BASIC Authentication .
  Now, comparing the results between two ESB applications, among which one requires the incoming request to pass through HTTP BASIC Authentication with the one which doesn't require this authentication, it reveals that the ESB service which is authenticating the incoming requests at HTTP Gateway of ESB service shows significantly less value for TPS field compared to the result from the other application which doesn't use authentication.
  Here are the results for a better comparison purpose.

ESB without security - 2000 tps
ESB with security - 600 tps

Why is there such a huge difference?

[1] TPS/BPS Calculation