For starters, the latest BC chapters reveal quite a few FTL-MFTL speed feats. However, viz mistranslated some key dialogue in chapter 375, and 376, which had an impact on the context. That distorted context led to the premature creation of this thread, which is understandable at first glance. However, after checking the sources it's become pretty clear that's not the case and that these were indeed mistranslations.
In chapter 375 the viz translation stated Lucifigus was controlling Morgen, but that's incorrect the kanji said "junijku" which is incarnated. Nacht never stated Lucifugus was controlling Morgen, he said Lucifugus was incarnated into Morgen, therefore Morgen is the one in control. Lucius himself stated that he incarnates devils into humans. Therefore, human souls are controlling the devil, not the other way around.
Furthermore, in chapter 376 the viz translation incorrectly stated Yami said "that attach was light speed, it's all I could do with black moon" Not only is that incorrect, but the true context is narratively consistent with the established speed scaling in the story. Yami never used Black Moon to defend against light magic in his second round with Morgen aka the latest 3 chapters, it's explicitly shown that he's using Black Moon defend against dark magic, not light magic. Yami has to focus on what to negate, it's not nullifying two spells simultaneously. The the more accurate statement is "Attack at the speed of light!!! I'm fully occupied/too busy defending myself with 'Black Moon'...!!"
Yami is fully occupied with with Black Moon since it's nullifying the effects of Morgen's Dark Magic within his mana zone, not light magic. We know this because in the spade arc Yami had a similar dilemma of choosing to whether to continue nullify Dante's presence of the demon king, or his gravitational singularity, but nullifying Dante's singularity with black moon would leave Yami exposed to presence of the demon king. Furthermore in chapter 368 Yami pre-power up was using black moon to nullify Morgen's light magic, but couldn't neg his dark magic.
Black Moon is a spell that combines Black Hole and Mana Zone. Black hole is spell that absorbs other magic, while mana zone is a technique that seizes control of the ambient mana in the area. The combination of those abilities allows him to passively apply Black Hole's spell nullification over an area, and create an entire zone that nullifies incoming spell that Yami focuses on.
Morgan amplified the magnitude of his light magic with dark magic. Therefore, Yami's concerned with physically deflecting a juiced up attack since his attention is primarily focused on . If he didn't, Morgen's dark magic would have pulled him in like last time and one shot him.Dark magic is a kind of magic that pulls things in with it's gravitation field. . In the cave arc we learned that very powerful light magic can overpower dark magic.
Later, It was explicitly stated that black moon wasn't nullifying Morgen's light, hence why Ichika used her Black Star to bend the light away. So they chose to combine their spell. Yami's Black Moon protects them from the gravitational pull of Morgen's dark magic while Ichika's Black Star protects them from Morgen light magic so each dark magic, is handling a singular spell, and their effects are amplified when they combine.
Not once did Yami say LS attacks were too fast for him, that would be narratively inconsistent given the feats in the latest chapters and past chapters. Morgen tries to blast Nacht with a ray of light, and Yami who was no where to be found instantly intercepts and deflects it away without the use of black moon.
That same Yami without black moon, also easily reacted to a confirmed LS blitz by Morgen with no issue and blocked it, while a base Nacht jumped out of the way to avoid the attack. The sfx and visuals of Morgen fading out with speed lines in the links above confirm he is using the LS movement spell (just like to Patry) Morgen is using light magic to propel himself forward. So these characters have no issue dealing with light speed spells in their base forms.
In the spade arc, Naalith (Lilith and Namaah combined) used their 4 wings to outspeed Devil Union Asta, despite being weaker than him. In a similar fashion, Morgen uses his 4 wings to propel himself faster and bullrush Yami without the use of light magic which means Morgan is than his spells due to reinforcement magic which is statistical boost. Light itself has a constant speed when traveling, while magical reinforcement does not, which is common. The speed of light is an inherent property in light rays, the only exception so far is demon-light magic which is corrupt and uses dark light.
Mages being physically faster than their own magic is nothing new since attributes have inherent properties such as light having the speed of c, and dark magic being slow. Yami uses reinforcement magic to compensate for slow nature of dark magic andBerserk Asta statued his own anti-magic slash to tag a teleporting 60% Dante, after just fired it.
Morgen broke through Yami's defenses with an FTL-mftl bullrush via reinforcement magic which causes Yami to say "he's too damn fast" Yami calls Morgen's too fast, not his magic. Morgen did not use light magic to propel himself in that scan. When a light mage uses light magic to travel at LS they engulf themselves in their element (light) and fly away at LS, as we've seen with Morgen, Patry, Lumiere and even Rhaia via copy magic. Morgen in the link above is not engulfed in light magic, he's just juiced up and using his physicals. He bull rushed Yami since he consistently evaded, and deflected his LS spells despite the fact he was primarily focusing on nullifying the effects of dark magic. So it would be incorrect to state Yami has issues with lightspeed when it required a speed that exceeds light to break his defenses.
Black Musha Ichika, Black moon demon god Yami and 4x union anti-magic amped Nacht are all relative in speed. However, that same Ichika has a clear FTL-MFTL feat towards the latter point of the chapter. Morgen bull rushed Yami to throw him off balance then launches LS spells less than 2 meters from Yami, and Ichika who is more than several meters away intercept them in an instant.
Lastly we see Nacht, Yami and Ichika match Morgen's combat speed created constellations across the battle field statuing debris once they can focus on the fight after activating Walpurgis Night .
The BC speed mftl speed meta has always been supported considering base Nacht in neverland could easily react to a LS blitz as well as base demon god Yami without the use of black moon. Morgen had to ditch his light magic and use magical reinforcement to break through Yami's defenses, and Ichika outpaced a LS attack from several meters away.