You get poor performance if you use the + operator on Strings repeatedly. You can write
String s = something + somethingElse + anotherThing + somethingDifferent + ... ;
and use the + operator as many times as you like, all in the same expression, and that is the right way to do it. There is an optimisation in the javac tool which sorts that out.
But using + on Strings in a loop slows down execution. You can actually see the delay if you watch after about 10000 iterations. To speed things up, find out about the StringBuilder(←link) class.
More about insert()
here.
Guess which index you want to insert repeatedly at the first position. Hint: it is the same as that for the first position in an array, so start by trying 0.