Incorporating an accelerated leveling pace is *definitely* something we want to incorporate in to the new addon My hope is to allow an option for you to set yourself to one or two higher speeds, where the guide will skip some steps along the way. The steps will have a special tag that lets the addon know to skip them if the person has that setting enabled.
Thanks for this guide. In the preparation section, you could add Goblin Gliders which are always super useful. I also recommend picking Northrend Engineering and Draenor Archeology as temporary professions. The first allows the use of the loot-a-rang toy which allows looting movs from a distance. The second one is required to picknsome treasures in Draenor.
Today, I finally found out what the real problem was. I have always had the "Enable Zone Map" feature off, but noticed that I had "Enable Auto Zoom" still enabled. I assumed if you turned the map off, the auto zoom would just automatically not do anything. Turns out, every time I moved, the auto zoom was still collecting information and 'zooming' a map that didn't exist. I've turned it off now and my game is perfect - not a single slowdown. The difference is dramatic.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.
The Horde 12-20 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped.  In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow.  Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images.  Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.

Level with a pet if you can because it will help keep you safe. If your pet is tanking all of the damage from the enemies you’re fighting, you won’t have to. The other reason is that pets are easier and cheaper to heal than you are. If you don’t have a pet, make sure you know how to use your defensive abilities. You can’t level quickly if you are dying all the time.
Players’ relationships with the NPCs around them have been hit-and-miss over the years. One of the original issues with Thrall, the Horde’s original Warchief, is that players began to refer to him as “green Jesus,” because of how infinitely powerful and infallible he was. But he’s been gone since the Warlord of Draenor expansion, and with the death of Varian Wrynn and Vol’Jin at the start of Legion, the old guard hasn’t been there to guide players the same way as it used to be.