Slowing the pace without boring the reader?

If you mean parts of the book where there isn't nonstop explosive-style action or fast-moving action, then I think that those are necessary every once in a while. I like a book where the pace is varied and there are moments when the characters can just talk for a second or the likes. It's got nothing to do with books really, but the thing that comes to mind is a quote from Alain Boublil and Claude-Michel Schonberg, the creative team behind the musical 'Les Miserables'. I remember them saying that they turned the Thenardier characters into the comic relief for the musical, whereas they weren't in the book, so that the show wasn't three hours of solid misery, as people wouldn't want to sit through that. Whether they achieved their goal is a matter of opinion but I agree with the overall reasoning.

Trick is, I think, to make sure that although those sections are slower that they are still interesting, in that they still advance either the plot or the characters or both in some way. Personally I find those 'breaks' in the action quite useful places to reveal secrets about the MCs, for instance.

I agree. That pacing slow-down should advance the plot. You shouldn't be slowing just for the sake of slowing. Another key is make sure your characters are interesting. If the reader likes your character, they are going to want to know all that stuff that you are revealing during the slow periods. If it doesn't advance the story, however, you've rambled too long and the reader is going to be thinking, "Get on with it!"

Slow-downs are good places to reveal secrets, foreshadow, and do all the necessary things that the machine-gun pace of action doesn't give you room to do. The difference between when it's intersting and not interesting--to me--is whether it's going somewhere. If it doesn't seem to be revealing anything, or giving me something useful, I get annoyed.

Provide some lower-level conflict. Raise questions you don't answer. Give us insight into why the characters are the way they are. Be charming. Be funny. Give the reader some emotions to play with.

Any or all of the above, but especially the first one.

I think the key lies in the distinction you raised at the end of your post: the difference between good writing and mediocre writing. Good writing does not need car chases and explosions to be interesting - mediocre writing will not be helped by car chases and explosions.

(Though they are pretty. )

If you're good enough - or willing to take the time to make your writing good enough, the slow parts should come naturally to you as you pace your material. Each story is different, so too is the pacing for each story. It's just part of good writing - and like all other aspects, the more you do, the better you'll get.

This is a good place to Tell instead of Show. Covering larger amounts of time in shorter spaces, paradoxically, has a slowing effect, so Tell us what happens over the next few days, weeks, months, etc. But, as has been said, the story needs to advance, and this is also a good place to foreshadow.