Using a different drive for Libraries and media is not about the speed of the drive, it is about the limited bandwidth the CPU can read/write to the drive. Video eats up that bandwidth a lot, and the OS and apps need bandwidth to function quickly and properly.
Just like your Internet connection, you can only upload and download so much data per second. Your hard drives have the same type of connection. There is a limit to how much data can be send up and down that wire per second.
Are all drives the same capacity?
Also, drive speed tests should be done at least 4 times, if not 6 or 10, and then averaged out for an "average" read/write speed. There is no absolute read/write speed for a drive. What manufacturers list on the boxes is a maximum possible read/write speed, which you'll never see in real world use.
Several things can effect the read/write speeds of drives. Bandwidth of hardware connections between HDD and CPU. How much data is on the drive. Block and sector size the drive was formatted with. OS accessing it. Format of the drive. Health of the Directory on the drive. Efficiency of a specific app's coding which is manipulating the data on the drive. Frequency of data being accessed by a specific app (more frequency means more RAM read/writes, which then get buffered to/from the drive's read/write process).
Your drives seem to all be in the same neighborhood, so I'd consider them pretty equal, and I think I could guess which ones have more or less free space on them. List the percentage of free space on each, see if there is a correlation.