The reason, why so great a stress was laid on the distinct reading of the Church Service, independently of its obvious necessity, was the general prevalence of an opposite practice amongst the Popish Clergy, many of whom, after they had conformed to the Liturgy, read it as they had been accustomed to read the prayers in their Breviary.* * The Clergy who read in this hurried and indistinct manner were called, in derision, "Mumble-Matins."
Source: wiktionary