In "Dulce et Decorum Est", how does the pace of the second section change?
Great question! You have obviously noted that there is a definite change in the pace between the end of the first stanza and the beginning of the second stanza. Note how the first stanza ends with a long sentence that rhythmically mimics the complete and absolute exhaustion of the sleepwalking soldiers as they continue staggering on their journey:
All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.
The effect is to emphasise the tiredness of the soldiers as they continue moving mechanically, on complete auto-pilot, towards their destination.
Yet note how the tone and pace is completely changed with the first line of the second stanza:
Gas! GAS! Quick, boys! - An ecstasy of fumbling...
Having been described almost as zombies, the soldiers are forced into sudden action by the arrival of gas. The repetition of the word "gas" and the description of the "ecstasy of fumbling" highlights the urgent necessity of getting the mask on in time to prevent the horrendous death that one of the soldiers suffers.
It is this juxtaposition, therefore, of a description of soldiers "cursing through sludge" who are half asleep with the sudden necessity for quick movement that changes the pace of the poem between the first and second stanza.