Then this happened
I'm really banking on Kuzan wavering leading to weakened reactions, or Garp is just much much faster than your average top tier, or a mix of both.
I think its a mix of both, Garp speed blitzed Marco who is a phoenix ofc. And thats at 80 and half of his strength/prime, according to himself at Beehive.
Emotional state definitely does affect your fighting capability and strength just like IRL. Martial arts movies/stories make a big deal out of this too.
If people deny this, they are blatantly biased and want to downplay certain characters for their own agendas or they really do not understand this concept or read the series properly.
Kuzan was emotionally conflicted, this was the most difficult battle of his life for himself aside from fighting Akainu to the death and taking part in Ohara, as far as we know. This is his lifelong mentor, his father figure presumably, the man who made him who he is now and made him become so strong and successful.
Anyone who is not emotionally stunted/impaired at all can see this. The guy is conflicted like Kizaru, Oda loves writing nuance like this when he wants to.
In a series themed all around AMBITION and thus conquering and dreams, fearlessness and such, naturally anything that makes you feel conflicted, will weaken your ambition and ambition is LITERALLY HAKI so your haki gets weakened too.
Oda outright told us this with Luffy vs Katakuri especially too:
https://www.reddit.com/r/Piratefolk/comments/17cnn9k