Who doesn't want to pick up a guitar and start shredding? Speed is one of the things that makes guitar fun. Getting that speed, however includes a technique that seems counter-intuitive.
Going slowly and messing up are the 2 keys to getting fast. Seems odd, no? There are good reasons for both though. Speed has everything to do with the connection of neurons in the brain and the time it takes for a circuit to convey a message. Now, think of your brain like a big hill of freshly fallen snow. Before you can really get moving in your sled, you need to carve out a path. This is why it's very important to go slowly. You need to make sure the path is perfect the first few times you take it. If you veer off course the first few times, anyone who follows behind you in their sled is going to take the same route. With whatever you're trying to get good at, take it as slow and and precisely as you can. Carve out the path. It's not time to sled full speed yet.
As you carve your path, it's important to be keenly aware of your mistaktes. When you mess up, always stop and correct. Don't continue on the wrong path, or that's the only path you will take. Stop and correct.
Once you ride down the path slowly a few times, you'll naturally get faster. You won't think about getting faster, it will just happen. The faster you get, the farther your sled will take you. As you progress, it's equally as important to stop and correct to make sure your path continues the way it should.
When you're working on a chord change or a new lick, you want to perform the action to where you're messing up about every third or fourth time. This is a good rate of progression. You'll be carving out your path and naturally be building speed.