But I think speed is an illusion because it can be very relative. A composer can write Allegro but the metronome marking is much slower. Likewise, the composer can write the slower tempo indication such as Andante but the metronome marking could be much faster. It really depends on what kind of metronome was used by the composer back in the day.
So I think speed is not as important as general musical feeling. When you are playing a piece, you have to transfer the emotion and the musicality that is built in into this piece. And if you are not ready to play this piece fast at the right concert tempo, then play it slower. You don't have to play it too fast because you if you are not ready for that, you will make more mistakes than usual and your piece will not sound as musical. It will sound like a race, like a sport.
In most cases, music is not sport. Of course, when you play scales, etudes, and exercises, there is some sport involved. But even then try not too play too fast.
Play as fast as you can possibly hear. That's the most important point here, I think. Listen to every single cadence, dissonant chords, important structural points, and modulations.
When you practice this piece, try to be aware of these things and the speed will take care of itself. That's because with time your fingers and your feet will become much stronger and the speed become more natural to you.
So it's better to think about these musical things, like how the piece is put together when you play. Try to really transfer your idea of analysis to the listeners.
Imagine, that you play something and you discover this modulation and find a nice dissonant D65 chord in the key of G major. You are not playing like a machine. You notice these things. You are feeling and leaning on the dissonances and hearing each voice and each part. By the way, that's why singing each part is so crucial when you practice. I hope you are doing that.
It's not easy, but well worth the effort because by doing it, you will learn to play musically and not like a robot. You will think like a real human being and transfer the thoughts and ideas that the composer has put into your piece. Remember this in your organ practice.