Originally reported by [~liach].

Attached Benchmark.java demonstrates that String.charAt is significantly slower than StringBuilder.charAt for the same String:

java -XX:CompileCommand=dontinline,Benchmark::test* Benchmark.java
CompileCommand: dontinline Benchmark.test* bool dontinline = true
testString() took 1363 ms
testStringBuilder() took 671 ms

Even with -XX:-UseSuperWord -XX:LoopUnrollLimit=0 -XX:-UseLoopPredicate -XX:-RangeCheckElimination the difference is significant.