String.format is slow compared to normal string concatenation (i.e. "foo" + "bar")! Bit me once when using String.format in a hot path.
https://stackoverflow.com/questions/513600/should-i-use-java...
https://stackoverflow.com/questions/513600/should-i-use-java...
Edit: i do believe you that + is faster, as it doesn't need to parse the format string, but you should still always do benchmarks with JMH or they won't be accurate.