Hello.

Since Yesterday, I notice that after giving telnet IP or SSH IP - it takes long time to display login: , I mean earlier after entering Server name It used to immediately ask for login ID and then password. But, now it takes min of 2-3 minute to ask for password.

Where can I check, whats going wrong?

I usually use strace to have an idea of what a program is doing. Of course, if you do not have some C programming background the output may not make enough sense to you. It outputs the system calls being called. You can look at the neighbouring system calls around places where large latencies occur to give you an idea what happened at that time.

One possibility that I encounter recently is that reverse DNS of IPv4 addresses is down or simply being slow. In that case, the SSH process may need to timeout before trying another one. MySQL has similar issues as well.

How to turn off DNS resolutions - First of all what is DNS, and why is it coming in picture here on UNIX? Direct telnet from server back to itself is also slow. I mean, it shows connected to server but login: prompt comes delayed only. There are no performance issues also on applications.

Please advice.

Exactly what happens to your system only you can trace yourself. Mine was confirmed related to DNS because that occurred on a supposedly idle server over a few days and afterwards the problem went away with no change made. That is a sign that DNS may be a cause. I could confirm with strace later that way.

For OpenSSH server side, there is a UseDNS configuration option in sshd_config. Set it to "no". Not sure about Telnet.

In fact this is in the OpenSSH FAQ:
OpenSSH FAQ

Maybe try pinging the IP and check its connectiviti.

what is DNS? and How does it impact to login process? What solutions are there to follow IF the issue is caused by DNS ?

DNS - Domain Name Server

This is the mechanism that a host asks another host to resolve a name.

A good test is to...

login to the server
note the IP address of your client
type "nslookup ip-address" and see how long it takes to reply.

/etc/resolv.conf is a good place to start.