I can tell you that I run a large (800,000+ lines of code) application, using restful webservices via Spring MVC, JMS, Atomikos transaction, Hibernate, JMX support, and embedded Tomcat. With all that, the application will start on my local desktop in about 19 seconds.
Spring Boot tries hard not to configure modules you are not using. However, it is easy to introduce additional dependencies and configuration that you did not intend.
Remember that Spring Boot follows the convention over configuration paradigm and by simply placing a library in your class path can cause Spring Boot to attempt to configure a module to use the library. Also, by doing something as simple as annotating your class with @RestController will trigger Spring Boot to auto-configure the entire Spring MVC stack.
You can see what is going on under the covers and enable debug logging as simple as specifying --debug when starting the application from the command-line. You can also specify debug=true in your application.properties.
In addition, you can set the logging level in application.properties as simple as:
logging.level.org.springframework.web: DEBUG
logging.level.org.hibernate: ERROR
If you detect an auto-configured module you don’t want, it can be disabled. The docs for this can be found here: http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#using-boot-disabling-specific-auto-configuration
An example would look like:
@Configuration
@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})
public class MyConfiguration {
}