Yes, Mitt Romney's speech against Donald Trump will slow down Trump's momentum because Romney is a well respected member of the Republican party. In his speech, Romney methodically detailed the flaws of Donald Trump's rhetoric and message, and passionately voiced that the election of Trump would spell doom for the United States.
Mitt Romney's speech against Donald Trump will slow down Trump's momentum because any Republicans who supported Romney during the last election who now support Trump may reconsider their statements. It is very powerful that the last Republican nominee denounces a current Republican candidate, so any Republicans may take Romney's words to heart.
I don't believe that anything will hold back Donald Trump, including Mitt Romney's speech. Romney probably prepared ahead of time to present some unfavorable information, but Trump's supporters will see through that. Trump is confident and opinionated. He will move forward in the manner that he finds is best for his campaign.
Some voters dislike or distrust Trump and his outspoken manner. They may not agree on Trump's foreign policies and income tax policies. Then there is the controversy about keeping Muslims out of America, which angered many people. Republican candidates have resorted to name-calling as a last-ditch effort to undermine each other. It does get ugly at times. Romney calls Trump a "phony" and a "fraud", It is a chance for Romney to get attention. Yet, he is doing so at the expense of another candidate, instead of spending more time highlighting important issues that matter to Americans, and convincing Americans that he is the best candidate for the job of Republican leader.