Why does the op amp output in this circuit have low slew rate?
Yes, it is common with cheap Chinese chips. You should also try to measure quiescent current and see how it differs from one that is stated in the datasheet. Chinese manufacturers usually tries to adopt chip design to poorer technology and it results in larger capacitances and higher quiescent currents. Since your op-amp is JFET-buffered it could be the case of a higher Cgs than it should be.