telnet & ftp response too slowly

We are using Solaris 2.5. When telnet or ftp to it, even via LAN or from itself, it respone very slowly at beginning.

For example. when I telnet to it, it takes 3 - 5 minutes until "login:" prompt cames out. But after the "login:" prompt, it becomes normally and reacts very quickly.

However, ping always response quicky. And there is no problem with DNS and routing.

Thanks in advance.