Here are the results from string concatenation in Java:
Standard concatenation (75 000 concatenations): 192.594s, string length, 300 000.
java.lang.StringBuffer-concatenation (75 000 concatenations): 0.062s, string length, 300 000.
RTI-concatenation (75000 concatenations): 1.765s, string length, 300 000.
The best in LS, 75 000 concatenations, 0.25s. The numbers aren't directly comparable, as I'm running on different hardware than Julian.
By "Standard" concatenation, I mean bigString += string. Standard concatenation and RT-concatenation is slower in Java than in LS. Using Java's native StringBuffer class, is a lot faster than any of the competitors in LS (so far).
I'm not surprised that Java's StringBuffer beats Julian's array-based StringBuffer-class in LS, as Sun probably puts a lot more effort into optimization of the language than IBM does with LS.
My test ran on Notes 7.02, which I think runs version 1.4 of the Java runtime. Newer versions of Java may be even faster.
My Java-skills are mediocre at best. Let me know if my test-methodology is wrong in any way.
>> Code for the test