The last time I saw this was on a slow-failing HDD.
Check a quick fsck might get you a few answers. You can find more info in the Linux manual. It could just be one or two bad blocks that you can recover and fix the problem (though, ofc, it’s time to backup your data).
The other, slightly unusual time I’ve seen it is with mixed RAM. 16gb made of 2x6g and then 2x4gb did some real odd things to the system. If it’s not the disk, and your box will boot with one stick of ram, try it to see if it fixes the issue. It could be that your RAM speeds are off (or your like me and just put two sticks you had lying around, and it basically worked until it didn’t).
An outlier, that I’ve not seen on modern machines is io/wait for a CD-ROM to spin up, even if your not accessing the CD-ROM. Normally caused by bad cabling. Based on the age of your machine, this is unlikely, but it might be worth unplugging devices to see if one is bad and not reporting properly.
This is, if course, assuming dmsg is empty
Final thought: see if your running SELinux. If you are, turn it off and try again. Those policies are complex, and something installed in a non-standard place could be causing SELinux to slow IO as it fills your logs with warnings.