He takes a step back at the warning to give Ianto space, glances around before looking up. His eyes widen slowly as he realises how quickly Ianto is moving, and then John is stepping forward again quickly read to grab Ianto in case he doesn't slow down properly. His hands snatch out at him as soon as he's in range, face furrowed in concern and irritation in equal measure.

"Jesus, Ianto! What the hell happened to taking it easy and not rushing?"

Apparently Ianto asked for his advice and then decided to just discard it. Not that John doesn't do the same thing often enough, but still. He does so with experience to guide him! It's different. Probably.