。
She often tends to talk fast.
Literal
She [topic-は] fast-mouth [with-で] talk tendency [subject-が] is-much.
傾向が多い ('tendency is much / often has the tendency to') is a slightly unusual phrasing — more standard would be 傾向がある ('has a tendency') without the 多い. The 多い here suggests the tendency is frequent or pronounced. 早口 ('fast speaker / fast talking') is a fixed compound: 早 ('fast, early') + 口 ('mouth') = 'fast mouth.' ~で here marks the manner of speaking ('with [a] fast mouth' = 'speaking fast'). 早口で話す is the standard collocation.