Just to take some mystery out of V8s behavior here. Builtins (e.g. Array#push and Array#concat) are mostly written in either a C++ DSL called CodeStubAssembler or a newer DSL on top of it called Torque. One of the Array#push implementations can be found here. These get statically compiled into platform specific code during build time, NOT runtime.
Some builtins (like Array#push) have a special handling in V8s JIT compiler Turbofan. If a function or a loop becomes hot enough, that contains such a call, it gets "inlined" into the JIT compiled code directly. This happens at runtime and the optimizing compiler can take advantage of information like type feedback.
Long story short, if you have a tight loop (often the case in microbenchmarks), Turbofan will throw everything plus the kitchen sink at it to optimize that code. The result is, that a builtin that does not have special handling (like Array#concat) might be slower in a microbenchmark (!) vs hand written code. The reason is simply that the builtin might have been statically compiled, while the hand written version was heavily optimized for one specific call-site.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.