It's speculation because the author didn't show the byte code or even just what the code decompiles to in Java.
But even with speculation, it shouldn't be that surprising that dynamic dispatch and reflection [0] are quite expensive compared to a cast and a field access of the length property.
But even with speculation, it shouldn't be that surprising that dynamic dispatch and reflection [0] are quite expensive compared to a cast and a field access of the length property.
[0] https://bugs.openjdk.org/browse/JDK-8051447