Choosing YAML over properties files is a readability and organization decision, not a performance one -- both are parsed once at startup into the same in-memory configuration.
Key Points: • Both formats are converted into a Spring Environment abstraction at startup; there's no ongoing parsing cost at runtime. • YAML's nested structure makes complex, multi-level configuration (like multiple profiles or nested lists) easier to read than flat dot-separated keys. • Properties files are simpler and less error-prone for small configs, since YAML is sensitive to indentation. • Startup time difference between parsing YAML vs properties is negligible for typical application sizes. • The choice affects maintainability and team preference far more than it affects the running application's speed.
Example: A configuration with nested profile-specific database settings for dev, test, and prod is much easier to scan in YAML's indented structure than as a long list of dot-separated properties keys.
Interview Tip: A concise interview answer is:
"The format choice doesn't affect runtime performance at all -- both get parsed into the same configuration model once at startup. YAML's advantage is readability for nested or profile-specific configuration; properties files are simpler for flat, small configs. It's purely a maintainability tradeoff."