ssh very slow connection

If creating the connection is slow, but it is normal speed after being created, you will most likely have a problem that the server is doing a reverse DNS lookup for the client and that, for some reason, it fails.

In general, when debugging this, you can also try to login from two terminals. With the first login look at the sshd log on the server, while you are trying to login from the second. That gives you more information about what the server is doing (or waiting for).

You can try to find proof for this for the cause being reverse DNS lookup by setting one, or both, of the following in /etc/ssh/sshd_config:

UseDNS no
UsePAM no

and see if that speeds up creating the connection. If it does you can often leave things that way until solved (if you care about that).

If this is a reverse DNS lookup problem, this depends on the DNS server the machine that you login to is using. According to Wikipedia not all IP addresses have a reverse entry, as this is not an actual standards requirement. But more likely this is some configuration issue.