According to the poem, those who serve God best are those who can bear his "milde yoak" patiently.
To the poet, God does not need "man's work" or gifts; instead, God looks for humans to wear his mild yoke patiently. The "milde yoak" here is an allusion to the New Testament verse in Matthew 11: 28-30.
Come unto me, all ye that labor and are heavy laden, and I will give you rest. Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls.For my yoke is easy, and my burden is light.
Here, the verse explains that God is a merciful and compassionate deity. He is neither a tyrant nor a cruel taskmaster; instead, the New Testament portrays God as someone who wants to provide rest for those who are troubled and weary from life's journey. Essentially, the poet comes to realize that God does not aim to "exact day-labor"; instead, those who serve God best are those who will wear his "milde yoak" patiently and those who "stand and waite" in God's presence.
Here, the poet is saying that God does not prize only those who move with "speed." He also appreciates those who can only "stand and waite." Since God needs nothing from human beings, it may be sufficient for those who cannot live the active life to serve him in quietness and contemplation.