That's one interpretation with another being that Sasuke inferred that Haku's speed was regressing through lack of chakra even though context reveals that Sasuke's ability to react to Haku's movements merely improved which is why Haku appeared to be moving slower, hence why Haku didn't even mention the need to conserve his stamina until after Sasuke awakened his Sharingan. That said, I'm not sure what the 2nd scan proves when Haku states that he needs to end the fight quickly in order to conserve his chakra and maintain his speed as a result, no where is it even indicated that Haku's speed had regressed either than the previous scan you presented which can be explained in an entirely different way which also makes sense given the manga's context.
Your interpretation isn't accurate with the feats Haku displayed in the WA. The version of Sasuke that is being discussed is not reacting to and blocking two kicks from WA Lee and Gai, especially not while flying at high speeds right into it, which makes Haku's feat even more impressive. At his peak, he was shitting on Sasuke and Naruto. At his peak, he was contending in a battle amongst Jonin tier and above. It is simply not applicable to put Haku on Sasuke and Naruto's level here. The DB gives him an extremely low stamina score. The manga states his KKG is extremely chakra taxing. It doesn't matter when the statements were made. We know that his speed was progressively regressing. Everything that I'm stating correlates and makes sense, while your standpoint insists that Land of Waves Arc Sasuke and Naruto can contend in a fight involving characters that are far above their league (WA 3T Kakashi, Gai, Lee, 7 Swordsmen of the Mist, etc).
Ok, then I'm not sure why you bothered replying. That's the main point.
To your first point; Those objects highlighted are moving. Look at Naruto's foot in relative position to the water bubble, it moves closer. The tinier water bubble to the upper left of the bigger one also changes position.
So a diminishing chakra reserve in Haku means that his speed goes from "Some of the fastest in the Naruverse able to blindside elite Jonin" to "Gets merked by children". That's an incredibly steep gap, and would mean that pound for pound Haku's chakra offers absolutely enormous benefits for small amounts.
All of this to say that I still don't think the argument makes sense. If the position is that Haku uses the mirror's reflection to transport, it would follow that you also have to state that Haku's speed can't diminish since light is constant (not sure if you believe this, just throwing it out there in case you do).
Because you're still downplaying Haku's speed.
Both those panels are drawn from different angles, but still didn't move until after Haku was shown finishing his assault. Kishi was clearly displaying all of Haku's movements while time seemingly stayed still.
I don't believe that Haku is moving at literal light speed movement, though real world logic isn't always applicable with a manga.