Typically, long start-up times are a problem of decision making, lifecycle choice, and requirements definition. As a project manager, you might not be able to solve all of these problems. But if you can identify them and point people to the causes and possible solutions, you might be able to start your projects earlier.
If it seems as if your projects never get started, your management might have a problem with decision making. Make sure your decision making includes all of these steps:
Defining the desired outcome, such as ranking the projects in order, so everyone knows the relative importance of each project.
Establishing the boundaries around the decision, such as chartering ...