Bags: This SHOULD be a no-brainer. The cheap-o version are Netherweave bags, but if you have the gold I recommend you get the largest bags you can afford, since it means you won't have to stop to clean them out as often. In fact, if you have an excess of gold, it's probably not even worth your time to stop and loot mobs that aren't quest objectives. This seems like a small thing, but over the course of 100+ levels, all those clicks to loot add up.
The zone you start in isn't under your control, in as much as it's pre-selected for you based on your race. But for leveling purposes we want to make scaling work for us as much as possible. That means sticking to the lower level zones. Even though scaling will increase the level of the quests and mobs in those zones, the layout and design of them is built for a player with no mount. So you'll blaze through them and gain more exp/hour than higher level zones which assume you DO have a mount.
As Blizzard calls it stats were "squished," and now players are finding it harder to level with that same scaling system. It's a two-fold issue, and game director Ion Hazzikostas isn't quite sure how to fix it yet. In a massive forum post several days ago he plainly explains the issue from the start, noting that "the feel and pacing of the level-up experience is a top priority for the team."
If you want to spend as little time on your alt as possible - just do Legion Invasions. You can use sites like (https://wow.gameinfo.io/invasions) to track when the next invasion will be. Log in, fly to the closest world quest, and once there - right-click on the quest's name in your quest log and "Find Group". Sign up for some group, or if there's none - just start your own immediately, and people will come to you. It is 10 times faster to do invasion quests in a group. Once that particular quest is complete - leave the group, and fly to the next one, where you should find another group. Don't expect the group from one world quest to follow you to the next. Then you simply log out until next invasion. You can do 1-2 invasions a day, and each invasion will give you about a level or 1.5 of experience, if you have full heirloom gear. Each invasion will take about 20 minutes.
Grinding mobs is tedious and slow. Questing is much faster since you get the massive quest experience in addition to the monster's experience. There is no grinding in this guide - you can go through every single level by simply questing! There is minimal traveling time as the guide leads you visually through maps so you can quickly rack up quest experience.
The dark side of this disparity is griefing; max level players are able to easily disrupt questing areas in use by players of the opposite faction, and can one-shot most enemy characters. Sadly, even guards in most areas are unable to stand against well-equipped max level players, especially as the current expansion progresses and gear steadily increases in power. Of course, the best answer to this is usually to recruit some max level players of your own faction, or switch to a max level alt, and make the griefer regret the day they flew into Goldshire.
Patch 5.3, like all new updates, is attracting a throng of returning players, many of which will be undergeared and willing to spend Gold to catch up with the rest. While the profits might not be as lucrative as with the new items, there may be less competition here. The key here is scanning the market for in-demand leveling gear and producing the right amount to avoid getting stuck with an unsellable stack of outdated items.