Selecting the "Parallel start" option may reduce the time it may take for an applications in a WebSphere application server to start. 

By default, this option is enabled. This option will cause the application server to start multiple threads. When disabled, a single thread will be used. You can either configure the entire application server to parallel start or you can configure a single application to parallel start.

Parallel start - Application Server

1. In the WebSphere admin console, select Servers > Server Types.
2. Select your application server.
3. Checkmark Parallel start.

Parallel start - Individual application

1. In the WebSphere admin console, select Applications > All applications.
2. Select an application.
3. Select Target specific application status.
4. Select an application server.
5. Checkmark Parallel start​.