Interesting experiment!
I don't think we actually have good data on that. Anecdotally, I also can't say I've heard people complain about Weaken for that reason. In fact, a lot of my students prefer Weaken to Strengthen.
They're fundamentally pretty similar in my eyes, so even though you were doing comparable difficulty questions, maybe there were just some trickier-to-read paragraphs or harder-to-interpret correct answers in the Weaken bunch.
Perhaps you go faster on Strengthen because it's easier for you to hear something that goes with the author's thinking.
For Weaken, I usually have the mindset of "GIVEN THIS evidence that I have to accept, HOW CAN I ARGUE the opposite of the author's conclusion?"
If you attack the answer choices thinking "WOULD THIS help me to argue the opposite of the author's conclusion?", you might find that some correct answers click more quickly.
Some correct answers just hurt the argument by attacking an assumption or undermining the trustworthiness or relevance of the evidence. In those cases, it's probably harder to quickly figure out which answer weakens.
But ultimately, it might just be a subjective thing that you vibe a little less with Weaken questions (for now! growth mindset, growth mindset)
good luck