In NNT Mel, Diane and Ban got together in 8 chapters.
Ban was first mentioned in chapter 7 or something, but he actually only met up with Mel in chapter 15 when they destroyed an entire dungeon with their arm wrestling match. More importantly, I think the dynamic here is a bit different. Mel was an experienced knight looking for his old friends based on clues he found and information that was given to him. Percival is a newbie that just set out on his first adventure and neither he nor Arthur's knights have any idea where to find the other 3 KOA or even who they are beyond vague descriptions. I think it's likely for the process of assembling the knights to be a bit slower for Percival than it was for Mel.
I think this idea also makes sense from a more meta perspective, since this time the titular group of knights only has 4 members rather than 7. Mel had a whopping 6 other knights to recruit, while Percival's only working with 3. You could argue that this means the KOA are going to be recruited in half the time, but I think it suggests each one of them is going to get twice the buildup and focus before they join. This also fits the idea that Percival isn't just recruiting his old friends like Mel was doing. He's meeting these knights for the first time and will have to befriend them before he can get them to join him. It won't be as easy as Mel having a short chat with Diane to convince her to come with him.
Additionally, something that kind of bothers me is that Percival still hasn't managed to defeat a single significant enemy. Ironside completely stomped him and the fox had to save him from Pellegarde to prevent the same thing from happening again. It could just be me, but it feels weird for Percival to be recruiting other knights before he as the main character and presumed leader of the 4KOA has even scored a single victory. His biggest feat so far is defeating some random wolf familiar with a slap, which seemed more like a gag to me. I want Percival to have a moment similar to Mel saving Elizabeth and sending Twigo flying into the sky. A moment where the inevitable anime can blast one of the signature tunes while Percival gets to look all heroic, the kind of thing that shows up in trailers and such. If another knight were to join before Percival gets an iconic moment like that I think he could end up overshadowing the main character. This is why I believe it would be best to wait a little longer to introduce another KOA.
Initial chapters were so good, these chapters are mid in comparison
I agree, but to be fair the initial chapters of a series are always loaded with new information since the author has to establish the setting and the characters. The pacing will necessarily slow down somewhat after that. Early chapters tend to be extra long too.