BBOD's can be caused by network or hard drive issues, it's merely the operating system telling you it's waiting for something that's taking a abnormal amount of time.
If it appears only while your onine, then suspect a network issue. Try other devices, connect Ethernet directly to the router/modem (power off/on) and try to isloate the issue. (like others on your network hogging bandwidth)
..WiFi, Internet problems, possible solutions
If it appears also while offline, then suspect hard drive damage, likely from bad sectors either that were on the drive when you got the machine (all have them) or via sudden shock if you moved the machine while it was operating. and the heads struck the patters ruining your data it's having trouble now reading.
It also might be your boot drive is filling up, thus it's having a hard time finding new space on the drive and breaking up files into tiny pieces. Reduce some large files like movies right away or your machine may fail to boot again.
Why is my computer slow?
I advise you backup your Users folders to a external drive as soon as possible, (in addition to TimeMachine because there is no way to verify the contents) as to really solve this issue may require a erase with zero fill and OS X install.
Most commonly used backup methods
Then go about repairing your issue.
..Step by Step to fix your Mac