Imagine reading a stranger’s work, a book you’ve never seen before, slowly and carefully. How long would that take you? I mean, how many hours? You can’t skim over descriptions of forest, house, or anything else you think might not be crucial to the story; you must read every word.
Imagine making sure you understand each phrase and sentence and paragraph. Envision writing a note with suggestions for rewording when something is unclear. Include writing out the three different ways you interpreted a particularly confounding sentence.
Picture yourself keeping a list of who wears glasses and has one squinty eye (and which eye it is), who has which pet, and who is childless.
What if you have to ensure all dialogue that needs a comma before the opening quotation mark has one? Ensure all question marks that should be inside quotation marks are, indeed, inside them? And make sure all the other bits of punctuation are where—and what—they should be? You’d have to pay attention to each piece of punctuation as you read the book. (Sure, you can use global search-and-replace for some of these tasks, but still, it takes attention and time.)
Imagine the writer has labored over the book you’re reading. (You may not have trouble imagining that!) What if you have to write about problems with the manuscript? You might take an extra bit of care with how you word your critique.
This is just a bit about my job, copyediting. Thorough reading, intense concentration, and careful wording of feedback are some of the reasons it takes the time it does.
Do you want to hire an editor—but the cost is daunting? Posts at the following links tell some things you can do to reduce the time and cost of copyediting: 7 Reasons Why You Should Read Your Book Out Loud by Joanna Penn and Preparing Your Manuscript for Your Copy Editor by Jenny Meadows. I hope these are helpful.