Jesus usually healed people instantly. I never stopped to wonder why this might be a more gradual healing. It makes me feel a little better at my lack of understanding sometimes when I think that the disciples had the same need for gradual understanding that I do.
The man's more gradual return to sight reflects the gradual journey of Jesus' disciples from spiritual blindness to spiritual insight. Surely they had seen Jesus perform many signs and wonders. Peter later confessed that Jesus was the Christ (Mark 8:29). Yet, as Jesus began to reveal to them what was going to happen -- that he would suffer and die -- the disciples struggled to comprehend God's plan for redemption. "And Peter took him, and began to rebuke him" (8:32). They still lacked the spiritual acuity to grasp God's plan for the salvation of the world through Jesus.The gradual healing also has been seen by Church fathers to contain even deeper meaning.
Mark: A Devotional Commentary
(The Word Among Us)
Allegorically (St. Bede, In Marcum), Jesus heals the blind man to announce the mystery of redemption. As God Incarnate, Jesus heals man through the sacrament of his human nature, here signified by his hands and spittle. This grace cures our spiritual blindness gradually, and, as with the blind man, progress is measured in proportion to our faith.
Allegorically (St. Jerome, Homily 79), the restoration of the blind man signifies our gradual increase in wisdom, from the darkness of ignorance to the light of truth. Christ's spittle is the perfect doctrine that proceeds from his mouth; it enhances our vision and brings us progressively to the knowledge of God.
The Gospel Of Mark
(The Ignatius Catholic Study Bible)