@SpringBootTest starts the full application context — all beans, all auto-configurations, all datasources, all message listeners. That is 10–30 seconds of startup time for every test class that needs it. Spring Boot’s test slices load a subset of the application context — only the beans relevant to what you are testing. Repository tests that once took 15 seconds with @SpringBootTest take 2 seconds with @DataJpaTest. This article covers all the major test slices and how to combine them with Testcontainers.