It might not stop them completely but it would slow them the first night.
( ( ( ( It ( ( might not ) ( ( stop them ) completely ) ) ) but ) ( it ( would ( ( slow them ) ( the ( first night ) ) ) ) ) ) . )
(ROOT (S (S (NP (PRP It)) (VP (MD might) (RB not) (VP (VB stop) (NP (PRP them)) (ADVP (RB completely))))) (CC but) (S (NP (PRP it)) (VP (MD would) (VP (VB slow) (NP (PRP them)) (NP (DT the) (JJ first) (NN night))))) (. .)))
It won't affect their speed at all. 
( It ( ( ( wo n't ) ( ( affect ( their speed ) ) ( at all ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (MD wo) (RB n't) (VP (VB affect) (NP (PRP$ their) (NN speed)) (ADVP (IN at) (DT all)))) (. .)))