There's missing debug output when using -vvv, which makes finding the cause of a problem harder, like below.

I often get a 5-second or 10-second delay *before* the connection. For instance:

0.000008 OpenSSH_6.7p1 Debian-6, OpenSSL 1.0.2d 9 Jul 2015
0.000071 debug1: Reading configuration data /home/vinc17/.ssh/config
0.000083 debug1: /home/vinc17/.ssh/config line 88: Applying options for ioooi
0.000092 debug1: /home/vinc17/.ssh/config line 374: Applying options for *
0.000099 debug1: Reading configuration data /etc/ssh/ssh_config
0.000107 debug1: /etc/ssh/ssh_config line 19: Applying options for *
0.000114 debug1: Hostname has changed; re-reading configuration
0.000121 debug1: Reading configuration data /home/vinc17/.ssh/config
0.000128 debug2: add_identity_file: ignoring duplicate key ~/.ssh/id_rsa-internal
0.000135 debug2: add_identity_file: ignoring duplicate key ~/.ssh/id_rsa
0.000143 debug1: /home/vinc17/.ssh/config line 374: Applying options for *
0.000150 debug1: Reading configuration data /etc/ssh/ssh_config
0.000157 debug1: /etc/ssh/ssh_config line 19: Applying options for *
0.000164 debug1: Control socket "/tmp/ssh-ioooi.vinc17.net-22-vinc17" does not exist
5.078375 debug2: ssh_connect: needpriv 0
5.078423 debug1: Connecting to ioooi.vinc17.net [92.243.22.117] port 22.
5.155833 debug1: Connection established.
[...]

even if a few seconds before, there was no such problem. When I run "ping ioooi.vinc17.net" during this test, there isn't any problem shown in the ping output (i.e. I get an answer every second).

So, I wasn't wondering what happened between "Control socket..." and "ssh_connect: needpriv 0". This was a mystery... until I tried with strace, where I could see things related to resolving, with a timeout in the connection to the DNS server just before the 5-second delay.