Starlight has an electricity limitation. She needs to draw power from a relatively nearby electrical source to fuel her beams and flash attacks. Once she's out of juice, she's down to her base stats and hand to hand skills.
Worth noting that A-Train wasn't actually moving at the speed of Starlight's beams when he fought her. It was that he was aim dodging her beams he zig-zagged towards her. But it's still a massive feat for him because he's so physically screwed up at this point that just these few seconds of fighting gives him a heart attack.