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Is polymorphism (significantly) slower than direct implementations?

So, I have this class, which is basically an implemented ArrayList, I was hoping that the code that uses this class would run faster, using java implementations, instead it works almost twice as slow. Is there an obvious reason for this that I am missing? Does this mean it is slower?

Original handwritten code:

public class VertexSet {
    private int[] _set = null;
    private int _sp;
    public final static int INIT_SIZE = 10, INC = 30;

    public VertexSet() {
        _set = new int[INIT_SIZE];
        _sp = 0;
    }

    public VertexSet(VertexSet ot) {
        _set = new int[INIT_SIZE];
        _sp = 0;
        for(int i = 0; i < ot.size(); ++i) this.add(ot.at(i));
    }

    public void add(int a) {
        if(_sp == _set.length) resize();
        _set[_sp] = a;
        ++_sp;
    }
    public int size() {return _sp;}
    public int at(int i) {return _set[i];}

    public String toString() {
        String ans = "Set: |" + size() + "| ";
        for(int i = 0; i < size(); ++i) ans+=this.at(i) + ", ";
        return ans;
    }

    public String toFile() {
        String ans = " ";
        for(int i = 0; i < size(); ++i) ans += this.at(i) + ", ";
        return ans;
    }
    /**
     * this method computes the intersection between this set and ot set.
     * @param ot - the other set
     */
    public VertexSet intersection(VertexSet ot) {
        VertexSet ans = new VertexSet();
        int i1 = 0, i2 = 0, a1, a2;
        while(i1<this.size() & i2 < ot.size()) {
            a1=this.at(i1);
            a2 = ot.at(i2);
            if(a1 == a2) {
                ans.add(a1); ++i1; ++i2;}
            else if(a1 < a2) {++i1;}
            else ++i2;
        }
        return ans;
    }

    private void resize() {
        int[] tmp = new int[_sp + INC];
        for(int i = 0; i < _sp; ++i) tmp[i] = _set[i];
        _set = tmp;
    }

}

Handwritten code after extending java implementations:

import java.util.ArrayList;

public class VertexSet extends ArrayList<Integer>{
    public final static int INIT_SIZE = 10;

    public VertexSet() {
        super(INIT_SIZE);
    }

    public VertexSet(VertexSet ot) {
        super(ot);
    }

    public int at(int i) {return get(i);}

    public String toString() {
        String ans = "Set: |" + size() + "| ";
        for(int i = 0; i < size(); ++i) ans+=this.at(i) + ", ";
        return ans;
    }

    public String toFile() {
        String ans = " ";
        for(int i = 0; i < size(); ++i) ans += this.at(i) + ", ";
        return ans;
    }
    /**
     * this method computes the intersection between this set and ot set.
     * @param ot - the other set
     */
    public VertexSet intersection(VertexSet ot) {
        VertexSet ans = new VertexSet();
        int i1 = 0, i2 = 0, a1, a2;
        while(i1<this.size() & i2 < ot.size()) {
            a1=this.at(i1);
            a2 = ot.at(i2);
            if(a1 == a2) {
                ans.add(a1); ++i1; ++i2;}
            else if(a1 < a2) {++i1;}
            else ++i2;
        }
        return ans;
    }
}
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