And let me re-ask then:
So, what exactly IS grade 8?
It is a mixture of things that cannot honestly be absolute. We know this because each new syllabus even today is coming with revisions of pieces that was in one grade that is now in the other.
Here's what I think.
There are 2 aspects which I think contribute to the grade of a piece.
The first which cannot truly be measured is interpretation. It's the ability to be convincing with your performance whether it's a diploma piece or a grade 1 piece, you need to make your performance believable. We often hear pieces that we say sound mechanical, or that we hear and can't quite put our finger's on it, but there's something missing. This is it.
We could find many examples of pianists that play wrong notes, play different tempos, are not always strict to the score, but we are still convinced of their performance, and feel the ideas.
The second is technical requirement. Now as technical requirements increase, interpretation get's harder to focus on. We can all close our eyes and play twinkle twinkle like a Chopin Nocturne, because there is relatively 0 technical ability required and so all focus can be on how it sounds.
Technical ability comes into 2 main categories.
The first is speed. The fact is the faster we have to do something, the harder it is to do. This is obviously why slow practice is required.
The second is refinement. It's the understanding of a technical requirement and how to apply it in different situations and making it as efficient as possible.
This may be why we find a piece that is much faster than another and is still in the same grade, because there may be less technical requirement, or that the requirements do not need to be very refined.
I've checked your wild horseman from what I can see, it's not in the 2017/2018 syllabus? It looks like it was 2011/2012... Refer back to my point on revising pieces and grades.
I don't believe a true database exists of all piano pieces and their grades so unless you've just found this on Youtube I wouldn't consider the grade labeling definite. Regardless let's take a look at what makes it an easier piece.
Firstly it's in C major. There are a few accidentals, but ultimately you can remain in the white key area and so there's little hand movement required. Further to this, in the first section, the lowest note is E and the highest note is only an F octave higher, which for most people all the notes in between can be hit with relatively little to no movement of the hand. No big jumps here..
All these notes are singular, so there's no double notes, no octaves, no chords at high speed. The tempo of the piece itself is relatively slow.
There's little to no dynamics, so we've completely taken out the interpretation requirement here, the few dynamic markings are on the beat of the music so there's no rhythm ideas to worry about either.
The chords that are in the left hand to start are very simple chords F major / E major / A minor / C major they almost all require very similar hand shapes and fingering.
Going back to the right hand they just look like broken chords / arpeggios things you begin learning straight from grade 1.
Ultimately this piece I think does fit into this grade, and I think maybe you're just concerning speed as the factor of difficulty here.
Unless maybe i've misunderstood and you think it's too easy to be a grade 3 piece? My points still apply but we need to compare it to another piece for you to see why it's grade 3 and not higher / lower
Topic: Getting to Grade 8 from scratch in one year.