Speaking clearly lets us absorb the dialogue
Novels are built on acres of dialogue. You can always point to a book like E.L Doctrow's Ragtime as the exception, but so very much talking pushes a story forward. You can help your characters tell the story by helping everyone know who's speaking at all times.
Where you place your attributions makes a huge difference. When you've got three people in a scene, and you try to put speaker attribution at the end of a paragraph, we'll get confused about who's speaking next
“It's to be done right now. What is his name?” Arthur hesitated and then saw that Merlin might not know about Lancelot's weakness.
“That will be the chief knight of the round table.”
The reader has now paused to puzzle through who is identifying Lancelot. Merlin, or Arthur? Be clearer, and save your reader the moments of puzzlement. All of the pieces of the puzzle are already on your page.
“It's to be done right now," Merlin said. "What is his name?”
Arthur hesitated long enough to see Merlin might not know about Lancelot's weakness. “That will be the chief knight of the round table.”
"You don't mean Lancelot?"
"None other," Arthur said. "I can see you didn't know."
The warning that you must heed: it's always clear to you, in your author's head, who's talking. Readers need help — to ensure that their experience equals your intentions.
As a rule, "said" is all you need as a tag for dialogue. It becomes an invisible word, leaving room for the names of speakers, or at least personal pronouns like he and she.