Could be more than one thing at issue here.

First, where on the planet are you? Closest major city is good enough.

Second, do you fertilize lawn late into the summer/fall?

No doubt the drought didn't do the tree any good and the fact most of the country had a very severe winter would also enter in.

The slow start and thinning top appears to be winter kill.

Fertilizer late in the season can exasperate this as the tree won't go dormant like it should in the fall, leaving the tree more prone to winter kill.

The black spots could one of those things that start to happen to a tree that reaches old age, certain bacterial infections can manifest themselves inside the tree and slowly grow over time.

Red maples lifespan is 80-100 years, so this tree could already be starting to decline. It may go on for a number of years but more of these problems may begin to become more evident, and will continue to as time goes on.

Red maple inner trunk bark can crack from freeze/thawing underneath the outer bark & you can't even see it. If so, large areas of or even the entire tree can be girdled. Hope this isn't the case w/yours.

Any chance there has been a lot of heavy traffic over roots in last couple/few years? Construction?

A disease like Phytopthera is causing this...

Did you figure out what caused this, I have the same tree doing the same thing?