Since infestors are support units as opposed to damage dealers, you're justified to question their usefulness without a strong force. I mean, you don't tech to them asap because you aim to out-tech your opponent's army the same way Protoss can go fast Void Ray or Colossus, for example.
Instead, I want them in this case for two reasons primarly
1. They allow me to transition smoothly into Ultralisks.
2. They are the most dangerous harassers in the game except for possibly unscouted Dark Templars. This is why I want them early, as two of them can take out whole mineral line of workers in 3 seconds using Fungal Growth. Fungal Growth is what makes them soooo powerful, as the workers can't escape. Once the fungal growth is down, the opponent has about 3 seconds to kill your infestors, or all those workers are gone.
One Fungal Growth will leave a worker at 9 health, so if you re-apply FG a few seconds after casting the first ones, the worker will die.
These are the two main reasons why I want to tech the infestors as early as possible. The fact that they are perhaps the best counter (with a lot of units in the front of them of course) in the game to massive units with Neural Parasite makes them even more attractive, so even if your opponent finds a way to deal with your harass, you still can't go wrong with infestors in your army.
At first I stuck to one base but when I tried FE, it actually didn't slow down the teching process too much. The only thing that slows it down is the fact that you have to build the extractor a bit later since you need all the minerals early on to afford the hatchery and the queen.
The downside of 2-base is that it's slightly harder to defend.and early aggression is stronger. Early on with this build, the larva isn't that important. In fact, you're going to end up with way too much larva to handle even with one queen. So why I would even think about going 2-base might not be exactly obvious. But if you spend the whole early game setting up for some good harassment, you'll have to make sure that you actually benefit from it in the end. What exactly do I mean with this? I mean that if your opponent has more workers than you do, even if you harass, there's no point in doing this.
Also as Zerg, playing with one-base is going to give you more resources than you can spend after a while. Not at first while teching but after a while. So giving your opponent complete map control is not the smartest thing to do since you'll need that second expansion before you even start to harass. This is the reason why taking that expansion before you even need it is a good idea.
So what's the BO?
After that, if you play against a Protoss and scout some early pressure, you get six lings (you'll have to extractor trick) and after that an overlord. Six lings + Queen should be able to defend your expo, as you'll throw down two spine crawlers when you can afford them.
Go lair as soon as you have 100 gas, build two more spine crawlers at your expo and build x amount of lings depending on your opponents army (you'll have to scout ofc). As soon as your lair finish, you want to throw down an infestation pit and another extractor.
You want to get to infestors asap so that they will have 150 energy as fast as possible. They have to regen 100 energy which will take about 180 in-game seconds.
We want to drop them with overlords (I'll explain why in a second) so we'll need the cargo and speed upgrade. The cargo upgrade takes 130 seconds to research so you want to start researching it as soon as your second infestor pops. After that, you want to get the speed upgrade when you can afford it.
When the upgrades are finished, put the infestors in an overlord along with perhaps a ling or two and send it to your opponent's mineral line.
As always, you'll have to improvise in order to deal with unforeseen events. If your opponent goes fast air, you might want to make a few less lings and a few more queens. Or you might even decide to go hydras instead of infestors to force him to stop making his air units and then continue with the infestor strat.
Infestors can burrow, why go with OL-drops?
I desire the flexibility of the OL-upgrades over burrow as a lot of people builds missile turrets just in case (especially if they see you fast tech). Overlord drops can't be countered with missile turrets or photon cannons unless you have A LOT of them. The only thing that can stop your infestors if you drop them on the mineral line is if he has units close to it or if he shoots down the OL before it even gets there.
Also, the OL-drops allow you to drop other stuff later in the game as well, making you hard to play against since you can strike anywhere.