WHY IS GNOME-TERMINAL TABS OPENING SLOW ?
If out of a sudden your Gnome terminal opening or New Tabs opening become very slow even though the machine has absolutely no load or the load is minimum, then you will have to check your .bashrc
Yes weirdly enough you might find you have added some program whose execution takes really long and this program is execute every time you run your gnome-terminal and if the program completion takes 7 / 10 seconds or more then gnome-terminal becomes pretty impossible to use.

Remove the bad program slowing down your gnome-terminal from .bashrc and if you need it to run everytime you login use Gnome native way to add the program to run on Gnome or Mate login. ENjoy 🙂