Since String is immutable in java, when you do a +, += or concat(String), a new String is generated. The bigger the String gets the longer it takes - there is more to copy and more garbage is produced.
Today's java compilers optimizes your string concatenation to make it optimal, e.g.
System.out.println("x:"+x+" y:"+y);
Compiler generates it to:
System.out.println((new StringBuilder()).append("x:").append(x).append(" y:").append(y).toString());
My advice is to write code that's easier to maintain and read.
This link shows performance of StringBuilder vs StringBuffer vs String.concat - done right
