Welcome to the land where the owners of the groove hang. There is only one thing that will get you where you want to go-- practice. Sorry, that's just how human bodies work. Remember that you are building necessary muscle for your task (there's a muscle in the middle of your forearm near the elbow that is growing by the day) at the same time you are making the neural connections that will allow your brain to control your hands. You are already seeing progress due to your dedication and joy. Keep it up.
"Fast" is only one part of the equation, though. You have to build up neural connections that will give you the necessary control as well. Control has to do with how hard you hit the strings, with which part of your finger and how deeply under/on top of your string. This is really, really important going forward, assuming you want to learn a second song at some point.
Here's the way to build those connections-- find a metronome and put it on a slow setting (try 60 bpm). Start with the E string and pluck it exactly in time with the metronome. Pay attention to how hard you are plucking. Keep at it. It will quickly feel boring and you will think you have mastered it. You have not. Record ten bars of your E string and then listen back. Is the pitch, timbre, volume and attack identical on each pluck? Probably not. Take note of what you are doing wrong and record another attempt. You will get better. Do the same with each string and then increase the bpm. This is a horrible exercise when you just want to play, but it will pay off in the long run. I hated my bass teacher for making me do this and I love him now.
Believe it or not, this is actually a shortcut to "fast". You won't get to the tempo ion Kryptonite as quickly, but when you get there you will sound good and you won't be wondering why you sound like crap even though you are going fast enough!
Good luck! May you be able to groove every day of your life.
"Fast" is only one part of the equation, though. You have to build up neural connections that will give you the necessary control as well. Control has to do with how hard you hit the strings, with which part of your finger and how deeply under/on top of your string. This is really, really important going forward, assuming you want to learn a second song at some point.
Here's the way to build those connections-- find a metronome and put it on a slow setting (try 60 bpm). Start with the E string and pluck it exactly in time with the metronome. Pay attention to how hard you are plucking. Keep at it. It will quickly feel boring and you will think you have mastered it. You have not. Record ten bars of your E string and then listen back. Is the pitch, timbre, volume and attack identical on each pluck? Probably not. Take note of what you are doing wrong and record another attempt. You will get better. Do the same with each string and then increase the bpm. This is a horrible exercise when you just want to play, but it will pay off in the long run. I hated my bass teacher for making me do this and I love him now.
Believe it or not, this is actually a shortcut to "fast". You won't get to the tempo ion Kryptonite as quickly, but when you get there you will sound good and you won't be wondering why you sound like crap even though you are going fast enough!
Good luck! May you be able to groove every day of your life.