I do have some old versions of Adobe Acrobat, Adobe Reader 9, and Adobe Digital Editions, but I have not used any of those recently. I can't find any information online about CEF Reader that doesn't refer to Adobe Creative Cloud, but I don't have that application. When I quit Microsoft Word and then quit Prezi Next, the slowdown went away, the hard drive stopped spinning, and the CEF Reader processes disappeared from Activity Monitor. Those were the only two applications running. This situation just occurred while I was running Microsoft Word and also the presentation software creator Prezi Next. If I open Activity Monitor, often I see that there are several processes named 'CEF Reader,' and some of them are using a high percentage of CPU time.

There are times when my iMac becomes slow and unresponsive, and I hear the internal hard drive spinning for no apparent reason.