Karthik, which authenication method are you using, Do you have any logging enabled, please check the logging file for reasons. You can use "debug aaa authenication" in the pix. I think this is not the problem of the speed difference. Check whether you have enabled http authenication on the PIX. If so please disable it, I hope it has to work fine then. Enabling http authenication on the PIX can cause problems like this.