Functional Java 8 new features aims to ease development. But we will see that you still need to deeply understand them internally, especially in the case of micro-benchmarking.
The original article from Tapiki is claiming that Java 8 Lambdas and Streams can be 5 times slower: Benchmark: How Java 8 Lambdas and Streams Can Make Your Code 5 Times Slower. Concluding that using functional programing may be a flaw in your code if you need performance.
But someone (ybonnel) proposed a pull request with many fixes on code (replacement of functions like Integer.max(), avoid unnecessary boxing/unboxing, etc.). Now performance is similar between Imperative and Functional programing (see : https://github.com/takipi/loops-jmh-playground/pull/2).
Conclusion: micro-benchmarking is a very difficult art in programming, especially with brand new features. Coding is particularly error prone since you need deep understanding to be fair.