For ages I've been a fast reader.

Lately I'm noticing I'm reading far TOO fast, and not taking in half
of what I read.

How can I slow down and absorb more?

Make notes!

This will help you to concentrate.

Hi,
   First choose a book or any reading materials that you will find
much intersting and in line with your field of interest. If it is not
effective, go back to basic, find a book with interesting pictures.

Read one chapter at a time.  Then set the book down and do something
else.  This way you can have a limited amount of information to work
with in your mind and your anticipation for the next chapter makes you
more attentive.

Speak the words out aloud, unless you read in public then perhaps sub-vocalise