As can be seen, Ren Fuji is commonly stated to surpass the speed of lightning, 440,000 m/s. He had surpassed the speed of lightning as early as the time he reached Yetzirah, and since then he's only grown faster. This is made clear soon after he achieves his Eine Faust Overture Briah form, where he explicitly states that his speed has increased over his Yetzirah state.
Later on, Marie takes control of his body. During this time period he gets a boost in speed that makes him 3000x faster than he was in Yetzirah.
Multiplying that by his previous speed of at least 440,000 m/s reveals that Ren Fuji was a minimum of 4.4c in his Eine Faust Overture Briah form after his Marie boost. But it doesn't end here. Later on, he gets another Briah form, Eine Faust Finale, which increases his abilities even further. At this point, he gets another speed boost...
...of 1000x. Bringing his speed to a minimum of 4400c. And from there it only goes higher.
Result
- Ren Fuji is Faster than Lightning - 4400c