With the following config (see config file below):
if only ethernet cable is connected, it 100% works: I can quickly SSH the computer via eth0's IP,
if both ethernet cable and wifi dongle are connected, it 100% works: I can quickly SSH the computer via eth0's IP or wlan0's IP,
if only wifi dongle is connected, I have to wait 1min30sec after boot to be able to SSH the computer! It's like the fact no ethernet cable is there "blocks" the boot* during 1min30sec.
Why does the fact no ethernet cable is present in case #3 block the boot for 1min30sec?
This is confirmed by:
# systemd-analyze blame 1min 28.442s networking.service 353ms getty-static.service
auto lo iface lo inet loopback auto eth0 iface eth0 inet dhcp auto wlan0 allow-hotplug wlan0 iface wlan0 inet dhcp wpa-ssid "<myssid>" wpa-psk "<mypasswd>"