Currently, the J-20 is under powered as the Chinese haven't completed development of the engines the airframe was designed for. So for now, anyway, it can't achieve the kind of performance it was designed for. What it does have going for it is a very low radar cross section from the front, but from the side and the rear, it is not so great. Certainly not at the level of US stealth platforms. The Indians reported tracking J-20's over Nepal last week with 4th Generation Russian Su-30MKI AESA radar, something that would not happen against F-22 or F-35's.
Thanks.