Q: Is it true that the decision will be conveyed faster if the manuscript is rejected?

The decision is processed faster in case of a desk rejection. This is because a journal pronounces a desk rejection right after the initial screening. A desk rejection does not involve the often lengthy peer review process and therefore the decision is taken faster. If the status of manuscript changes to “Decision in process” without changing to “Under review,” it is clearly a desk rejection.

However, a manuscript can also be rejected after peer review. In such cases, rejections are not faster; they take as much time as any other decision.

Related reading:
 

• What does a change from "With Editor" to "Decision in process" mean?
• Does a direct 'Decision in Process' status indicate rejection?