OpenJDK 11

Benchmark           (size)  Mode  Cnt       Score      Error  Units
Fibo.FiboRecursive      40  avgt   25  922753.617 ± 2747.036  us/op

0.9s, basically.


OpenJDK 17

Benchmark           (size)  Mode  Cnt        Score      Error  Units
Fibo.FiboRecursive      40  avgt   25  1032010.482 ± 8309.430  us/op

1s, basically.


OracleJDK 23

Benchmark           (size)  Mode  Cnt        Score      Error  Units
Fibo.FiboRecursive      40  avgt   25  1082715.628 ± 6358.441  us/op

I would expect your numbers to be dominated by JVM startup. Are you on SSD?

At the same time, fibonacci using GCC is 0.38s, and using clang is 0.56s, so it's not just a slow computer.