Byte Buddy is quite efficient. It is more or less like implementing Java code per hand. Byte Buddy comes along with a benchmark where the results are listed here: http://bytebuddy.net/#/tutorial
Of course, there might be a flaw in the benchmark, those are tricky. Si please challange it, I am always interested in such details. I also guess, that the advantage will decrease with future JVMs.
This way, it even beats the use of MethodHandles for accessing fields which still have a slight overhead over "normal" field access: https://gist.github.com/raphw/881e1745996f9d314ab0
Of course, there might be a flaw in the benchmark, those are tricky. Si please challange it, I am always interested in such details. I also guess, that the advantage will decrease with future JVMs.