ADDITIONAL SYSTEM INFORMATION :
I have tested it on various versions of Window and got the same results. I have not tried any other OS.

A DESCRIPTION OF THE PROBLEM :
Basic string concatenation performance has got significantly worse in Java 11 compared to Java 8. I wrote some very basic test code and posted on Stack Overflow to see what other developers thought and they confirmed similar results.

https://stackoverflow.com/questions/55395958/java-11-string-concatenation-performance-vs-java-8

REGRESSION : Last worked in version 8u192

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the code in the above link under Java 8 and Java 11.

https://stackoverflow.com/questions/55395958/java-11-string-concatenation-performance-vs-java-8


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code will take noticably longer to run under Java 11 versus Java 8.
ACTUAL -
The code takes noticably longer to run under Java 11 versus Java 8.

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https://stackoverflow.com/questions/55395958/java-11-string-concatenation-performance-vs-java-8

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CUSTOMER SUBMITTED WORKAROUND :
I have not been able to find any runtime flags or alternative string concatenation packages that give me a comparable performance to Java 8.

FREQUENCY : always