There are a lot of academic studies available in the literature addressing this topic. In summary and in my own words, it is a two-edged-sword. This is due to a few primary reasons. First is how the two company cultures differ. Which includes how decisions are made. For example, if some managers at the target could make decisions and others at the acquire had to get approval, conflict and stressors can quickly affect the managers opinion on what is acceptable to them and how things are done the wrong way.
Another concern is if the M&A was unwanted/hostile, some managers could purposefully want to negatively impact the acquire’s performance. This might seem counter intuitive, because this could mean not performing well and being let go. However, if there are negative emotions connected to the M&A, the manager might gain personal gratification from undermining the growth. They might even consider “jumping ship” to a competitor to cause further pain to the acquire. This doesn’t sound good but it does happen.