Why is my iMac (late 2013) HDD so slow? My iMac's hard disk has been slow from the start. But recently it is becoming unbearable. Disk read and write speeds are consistently below 50 MB per second, without any applications producing any CPU load. 

I do see that mds uses lots of disk I/O. Could this be the problem? If so, what can I do to alleviate it? I tried turning off the indexing with sudo mdutil -a -i off, but it did not help much. 

How else can I debug this?
To finish this question: I had opened a bug report with Apple. They determined that a botched Aperture photo library migration lead to excessive disk I/O. In the end, I reformatted the whole Mac. Plus to make the machine more useable, I attached a 256 GB Thunderbolt SSD and created a FusionDrive, as per this blog (in german). There are lots of howtos explaining how to create a FusionDrive. This really gave the machine an incredible performance boost.