Complex sentence: The Su-15 was optimised for the high-altitude interception role with its fast climb-rate and high speed at high altitude but lacked look-down/shoot-down capability, even with the Su-15TM's more sophisticated Taifun radar.
Split into simpler sentences: The Su-15 was optimised for the high-altitude interception role with its fast climb-rate and high speed at high altitude. It lacked look-down/shoot-down capability, however, even with the Su-15TM's more sophisticated Taifun radar.