While this is an understandable complaint, I feel like it holds up well enough on its own.
We see the swords start to swing, we see the lightning emerge from the cloud; and we see the sword strike complete concurrently with the lightning strike. So either it was "slow" lightning (though do we not tend to treat normal-looking lightning from the sky as the genuine article?) or Gil's swing speed is proportional to lightning over the smaller distance it needs to travel. Another interpretation is that Gil simply triggers the lightning to strike at the moment he completes the swing, but that doesn't change the fact that Meliodas easily counters said lightning.
As you have pointed out, Gil then dodges the reflected lightning, which further supports that he is that fast.
Later, we get a very clear look at Meliodas staring at a lightning bolt from the sky with enough time to counter it.
https://imgur.com/a/tf8vT
The manga never gives any other objective reference points for speed. If we throw this out, even Galan's teleportation-dodge feat is useless to us, because the only thing we know about the speed of teleportation is that it
seems instantaneous to the likes of Meliodas. You either have to accept it or not.