I've always found that my favorite kind of worldbuilding is the kind that only makes itself known as it's needed for the story. I know it's a film (based on a book), but Hayao Miyazaki's Howl's Moving Castle does a great way of illustrating this. The world feels vast--throughout the movie it becomes known to you that there are two different kingdoms who don't like each other, the prince of one of them has disappeared and it eventually leads a war, there are lots of wizards in the world that the head magician one kingdom is essentially enslaving and using as instruments of war...but you aren't told any of this upfront. You're just sort of dropped into the world, and you're given the information bit by bit as is necessary.
And not all of it is spelled out clearly. A lot of the news you hear about the prince disappearing and the escalation of the war comes from what people in the background are saying, and you only hear about that when it becomes necessary. And not only that, the knowledge is put into context almost immediately. For example: the same scene in which you learn about Howl's old teacher is the same scene in which it's decided that Sophie is going to confront the teacher for him. A new character is introduced, and immediately they are made a part of the story's present events.
Had the teacher been alluded to earlier in the story, it wouldn't have made much sense, and it would have felt like a waste of time because you're made aware of this person but nothing is done with it. It's the difference between being introduced to a person at the beginning of a party and not seeing them again until toward the end, and being introduced to someone and immediately having a dance with them. In the first, you're just going to forget about the person and until that second meeting their whole introduction will have felt quite banal.
But that's just one approach to revealing worldbuilding. I guess my point is that from what I've noticed anyway, worldbuilding feels like a chore to the reader when it isn't in context. I couldn't get into the Shannara books by Terry Brooks for this reason. You spend the first chapter or so meeting this boy and this druid and having the story told about them, but then there's a point where the boy and druid sit there and the druid goes on for ages about these ancient wars and civilizations that have essentially nothing to do with the story at present. Perhaps it all makes sense later, but until that bridge between the past and the present is built all that history is totally unrelatable and just feels like you're reading a history textbook with stranger names and places than usual.
Tolkien does this a lot in LotR, but I'm more forgiving of it there because while it still often comes across as a major information dump, at least there is always a bit of context for it. Frodo will have asked a question about something, or they'll be in an area that is historically significant and then somebody goes and explains why. Again, it's still a bit TMI at times for my tastes, but it never is just arbitrary.
Apart from that, I don't really have much advice I can think of to keep a middle from dragging. I feel like I would need to know more about your story, but I don't have time to read a manuscript and you probably shouldn't be posting such a detailed outline of your story for the whole world to see, so we'll just leave it at that, lol. I guess maybe just find someone who is good with stories you trust and have them read your manuscript, if you have anybody like that you aren't already using to bounce ideas off of.
