Depending on the task Java is actually faster than C++ at times. Garbage Collection, when done right, can for example be faster than manual memory management. And a JIT compiler can do optimizations that a normal C++ compiler could only do with help from the developer.

There is a reason many of these kind of systems are written in JVM based languages. Examples: Hadoop and all of its siblings, Cassandra, Storm, Kafka. So either all of those people in successful projects make "questionable decisions" or your knowledge of Java/JVM performance has been outdated by the new developments of the past few years...