This portal is still operating post-Shattering and is located in the Violet Citadel, taking you outside the Caverns of Time in Tanaris. From here, the world is your slightly-pain-in-the-ass oyster.
The secret? Silithis! I kid you not. Take either the flight point slightly south-west of the CoT cave entrance or the Gadgetzan one. When you get off the AFK flight point at Cenarion Hold, turn on low-level quest tracking and grab the quest from the faction-specific NPCs standing in front of the Blasted Lands portal; once you do, that portal will become clickable:
Take the portal to Blasted Lands, and make sure to abandon the quest. From here, it is a short ride through the Dark Portal for TBC content... or you can click on the handy Stormwind portal on the other side. BAM!
Horde, as usual, has the better deal in this case. There is a portal to Orgrimar across from the Stormwind portal, but they get the added bonus of being able to head to Shattrath, where the portal to Isle of Quel'Danas still
Does any of this actually save any time? Yes! And no. It actually depends to what degree you may or may not have missed the boat. Boats dock for exactly 60 seconds before they head off, and it takes 3:25 before before another Northend boat docks again. The following assumes that you just missed the boat, as that feels like what always ends up being the case:
CoT Portal --> Silithus --> Blasted Lands --> Dark Portal --> SW = 7:30
Dalaran --> Borean Tundra --> Boat --> SW = 10:17
CoT Portal --> Theramore --> Boat --> Menethril Harbor --> IF = 11:35
If you end up getting on the boat just as it is leaving in a sort of best-case scenario, you can make it to Stormwind in just shy of six minutes the straightforward sort of way. All of the above is under the assumption that you are moving at just 100% speed on your mount. If you end up moving faster due to talents or Crusader Aura, the CoT portal path ends up being faster than the boat as there is that initial mounted stretch going to Gadgetzan and again in the Blasted Lands.