问为什么使用焊接接头会使我的Box2D程序变慢？

EN

for(pos = _contactListener->_contacts.begin(); 
    pos != _contactListener->_contacts.end(); ++pos) {
    MyContact contact = *pos;


    b2Body *bodyA = contact.fixtureA->GetBody();
    b2Body *bodyB = contact.fixtureB->GetBody();

    b2Vec2 worldCoordsAnchorPoint = bodyA->GetWorldPoint (b2Vec2(0,0)); 
    b2WeldJointDef weldJointDef;
    weldJointDef.bodyA = bodyA;
    weldJointDef.bodyB = bodyB;

    weldJointDef.localAnchorA = weldJointDef.bodyA->GetLocalPoint(worldCoordsAnchorPoint);
    weldJointDef.localAnchorB = weldJointDef.bodyB->GetLocalPoint(worldCoordsAnchorPoint);
    weldJointDef.referenceAngle = weldJointDef.bodyB->GetAngle() - weldJointDef.bodyA->GetAngle();
    weldJointDef.collideConnected = true;
    weldJointDef.userData = @"tile";

    weldJointDef.Initialize(bodyA, bodyB, worldCoordsAnchorPoint);
    world->CreateJoint(&weldJointDef);
    //}
}