Voluntary XIII

The sheet music I have for this piece indicates to play it "slow". This piece strikes me as a bit dolorous, and so I feel it has to be played at a speed that permits the dolour to come through. So, I wonder how slow is "slow". I struggled with the tempo in this piece. Playing it too slowly seemed to make it overbearing, especially considering that one is supposed to play it with "full organ". But too fast and it seems to lose its melancholy character. If anyone has insight into this, I'd appreciate comments.