Watching videos of people going stupid fast downhills is both inspiring and discouraging. Most of the time, when you see something like that, you're probably wondering, "What do you even need to do something like that?" To tell the truth, you don't need much. For a decent longboard setup meant for going fast, you'll need a decent setup and some safety gear.
A Good DH / Freeride Oriented Deck
This is the first thing you'll want to really get your hands on. A lot of the time, you're going to build the rest of your setup around this. A good deck menat for going fast is going to be a topmount, stiff, have good concave that grips your feet, wheel wells so that you can use larger wheels, and (typically) multiple wheelbases. When looking at wheelbases, consider distances anywhere from 21" to 26". Shorter wheelbases are going to be more nimble, while longer ones are going to be a bit slower to respond. Any shorter than 21" is going to be a bit too squirrely, and anything longer than 26" is going to feel like a boat and take too long to respond.
Griptape is something to consider when it comes to skating fast. Many boards come gripped already these days, and that grip is just fine. When you're just starting out, you don't need coarse grip right off the bat. If you're gripping your board yourself and you're serious about going fast, get some coarse griptape, otherwise, just get a sheet or two of the standard stuff.
Downhill Oriented Cast Trucks and Wedges / Risers
Longboard trucks meant for going downhill are universally going to be RKP, or reverse kingpin, trucks. The geoometry of these trucks allows for more stability. Most skaters are going to recommend a front truck that is anywhere between 44 to 50 degrees and a rear truck that is 35 degrees or lower. A higher degree truck in the front produces more turn, while a lower degree truck in the rear allows for more stability. Most cast trucks don't come in splits like this, but there are wedge risers that allow you to fine tune your truck geometry. Getting a variety of wedges is always a good idea, but to begin with, grab a couple 7 degree wedges to start.
Truck width should be somewhat narrow. For a beginner, a maximum width should be about 9" (160mm), but the range is really more like 135mm to 160mm. Anything wider and you'll likely drift around a bit, and anyhing narrower is going to be tricky to set up when you're just starting out.
Weight Appropriate Bushings
Chances are you're not going to be the right weight for the bushings in your trucks. You can make do, of course, and many skaters have done this in the past, but it can get very janky to do so. For a decent setup, you'll want to make sure that you have weight appropriate bushigns for your trucks. Many companies, like Venom, Riptide, and Seismic, have charts that allow you to see what bushing durometers are good for what weight ranges. Find an appropriate duro, and then pick up those, a duro softer, and a duro harder in a double barrel configuration. This is going to allow you to dial in your trucks, making them very skateable for speed.
70 to 75mm Wheels
I am not someone to really harp on what wheels a person is riding, but when you're just starting out, you'll want to get some good 70 to 75mm wheels. They don't have to be expensive (check out budget brands like Blank Wheel Co, Banana Wheel Co, or used wheels from the B/S/T), but they need to be serviceable. For beginners, a square-lipped "race" wheel isn't necessary, but they can give you a little bit more confidence through corners. A round-lipped wheel that has a stone ground surface is going to be a little bit more slippery. Generally speaking, start with a square-lipped wheel as you're getting used to your setup.
"But what about bearings?" I hear you, but honestly, they're not going to matter when you're just learning. Grab some Zealous built-ins or some Bones Reds. Don't buy expensive bearings, it doesn't matter.
Helmet and Slide Gloves
This is the absolute bare minimum when it comes to safety gear. A helmet is going to save your dome in the event of a bad fall, and gloves are going to save your hands when you eat it in a forward direction. Learning to skate faster has its risks, and these are way to mitigate those risks. Optional safety gear inludes elbow pads, knee pads, and wrist guards. These aren't a bad idea, but not everyone finds them entirely necessary. It's really up to the rider on those.
Understanding Cost
One of the big barriers to entry in skating fast seems to be cost. Many of the popular, advertised boards for going fast are fairly expensive. When you really start digging, you'll also find that many people skating REALLY fast are on precision trucks, ceramic bearings, and fresh wheels for every other run. You don't have to have a lot of cash to get into DH / freeride skating. Buy used, buy budget, buy thrift, and check out deals when it comes to your setup. When it comes to safety gear, don't pinch pennies - get a new helmet that fits your dome.
