I have just copy and pasted an issue on the forum because I have the exact same problem! Someone private messaged the uploader:
I have been using Adobe Digital Editions on my Mac for about two weeks to read a school textbook I downloaded. I have to restart the program every 10-15 minutes because it eventually gets very slow. Every time I try to highlight a text, make a note or even scroll the page it takes 30 seconds and I get a wait signal. Once I restart it, it begins to function normally again. Any idea why it is being so slow? All of my other applications on my computer work fine and have no issues.