Jini security framework that is being maintained in Apache River project requires roughly 3 sequential tasks in every connection to cope with security threats in communication between clients and servers in the IoT sensor application. These tasks are sequentially proxy preparation, certification of identity and its credential for mutual authentication, and cryptographic operation for session encryption. Since the proxy preparation task is time spent on preparing a secure and trustable stub for both client/server sides, it is not one of the substantial communication-delay factors. We propose a method of improving proxy preparation reducing service connection delay by completing the preparation of mutual authentication data and session key while on proxy preparation task. Through experiments on a test-bed, we have confirmed that service connection delay time can be reduced by about 2.6 times in proportion to the message sizes through our proposed method. Our work presents an approach to reduce the additional overhead that would be paid for by applying the Jini security framework to the development of sensor application based on Jini IoT.