So I recently got into fingerstyle guitar and learned to play Someone Like You (sungha jung) and it took me a couple months, which I think is too long. But when I started looking for another song to learn I realized that I have difficulty preforming many difficult things like a thumb slap and even some chords I cant stretch to and play cleanly. I also see people who are able to pick up a guitar and just start playing some tabs well, which I can't do. So I was wondering what should I be practicing to learn the basics/intermediate level of guitar so I can learn songs quicker?
"have difficulty preforming many difficult things like a thumb slap and even some chords I cant stretch to and play cleanly"
Seems to me working on your weak spots is always a good place to start. However if you can't do those things while not fingerpicking, e.g. While using a pick,there is no way you'll be able to do it while adding more complexity in your right hand.
Finger picking is mostly a challenge in the right hand, so if you can play some chord changes smoothly with your left hand you, maybe just take C, F and G for example and then arrpegiate the chords with the fingers. Since the left hand is already solid this will help you establish a solid right hand.
There are certain patterns you can learn with the right hand (google travis picking) that you can practice first on one chord then on a simple change and then once that pattern is mastered in the right hand you can now play that pattern while working on more complex stuff in the left hand.
That would be my overall approach, if you can play something easily with one hand then play that and work on something more difficult with the other. If you are working on a new picking pattern play it with simple chords changes or one chord and then increase the chord complexity. If your working on a new left hand technique, like stretching, then play it with a picking pattern you have already mastered.
There is sort of a duality to learning music. On the one hand you would benefit from a systematic approach to learning the instrument and mastering technique. This is really critical and in my opinion anyone who wants to learn an instrument will benefit from lessons, working through the Mel Bay Series, Carcassi, Levitt, etc. On the other hand the thing that inspires us to play is the desire to play a favorite song. Typically what we want to play is way too difficult for our skill level. Yet the desire to try keeps us coming back.
It is not wrong to learn technique by by learning a song that uses the technique but there is a danger that you will sacrifice proper form to get through the song and that can lead to problems years later that are difficult or impossible to correct. I think you would benefit from working through some type of graded system for finger style guitar either on your own with videos, books, etc. or preferably with a teacher who can observe you and correct technique.
As for taking a couple months to learn that is not too long and you need to gauge your expectations. I've been playing for over 40 years and it still takes months to learn new pieces, perhaps over a year to get it clean. Of course this depends on the nature of the piece and do not know the song you are referring to.
In theory, if you learn basic exercises in the beginning that will translate to easy performance later. You will pick up new songs quickly because you are not trying to learn the technique from the song. However there will always be cases where a new piece of music has an unusual passage, or a new technique or approach that you are unfamiliar with. Thus you will always be adjusting as you go.
If you are serious about finger style guitar you should work through a few method books. There is a set of exercises in the classical guitar world that can easily be adapted to bluegrass style picking or other techniques, "Papararo | The Technique of Arpeggios for Solo Guitar". In a nut shell the first half of the book covers right hand picking patterns with the thumb (p) on the three bass strings and the three fingers (i, m, a) on the treble strings (G, B, E) respectively. The exercises are sequences of picking patterns like (p, i, m, i) or (p, m, a, m, i) etc. Just about every combination of fingerings is in there and the goal it to get clean, smooth, controlled, execution of the patterns at very high speeds (e.g. 16th notes at 180 pbm) without galloping or loosing the beat and without forcing it (should be able to play fast pianissimo as well as forte). The unique thing about these exercises is that they are performed on the open strings, no left hand chords or fingering of any kind. The picking hand is completely isolated. Near the end of the second half of the book there are actual arpeggios and melodic phrases that apply the right hand technique. It's a great warm up and I usually to one page at random each day as part of my warm up routine. Of course that's after years of working through them. Another set is the Giuliani exercises (not by the former NYC Mayor). These involve simple changes like C --> G7 --> C, etc. So you are holding a chord and executing a simple cadence while applying the right hand pattern.
Since you are asking about right hand specifically I wanted to point out this set of exercises. But the idea is that such approaches to all guitar technique exist in classical and modern guitar. Investing some time in them pays dividends and those people who can "...pick up a guitar and just start playing some tabs well..." have either been doing it for a much longer time than you or invested in these exercises.
If it is taking you months to learn a song, that means the song is too hard for your playing standard now, this is a very common mistakes when people are learning.
You should start with easy arrangements and work your way up.
Here are some websites that I find useful for learning fingerstyle guitar: