Spring entity to DTO conversion

Suppose I have a class A like:

public class A{
    Long aa;
}

And a Jpa Repository been extended on it. Now ARepo extends Jp...<A,Long>

Now I have have another DTO B like:

public class B{
   Long bb;
   Long cc;
}

I use this DTO once in a while, that's why I don't directly save it in DB.

Now what I'm doing is:

List<A>as=aRepo.getAll();
List<B>bs=new ArrayList<>();
for(A a:as)
    bs.add(new B(a));

I don't like this and it seems real slow, how to resolve this ?