Why did the speed of the RAID array drop?

Question

RAID

There is a file server on Windows 2012. 6 HDDs are connected to it (WD Black - system, Seagate - 4Tb - out of raid, WD Red 5Tb - out of raid, and 3 WD Red 3Tb combined in RAID 5). There is a file sphere on all disks, users constantly work with them over the network, write data (On the same server, a domain controller, RDP server and little things). Everything worked fine for several months. The read speed of CrystalDiskMark on drives not in the raid was on average 150 MB/s, on the raid about 250 MB/s. Suddenly, about two weeks ago, the speed on the raid dropped to 60 MB / s.
What has been done:
- Hardware RAID, motherboard Asus P8H77-V LE, the built-in utility says that the raid is normal. That is, in fact, the raid did not switch to degrade mode.
- SMART checked, disks are normal.
- Other drives are running at normal speed.
Question. What could be the reason for the sudden drop in speed? What are the solutions?
PS I know that Raid 5 is not the most productive, but 60 MB / s is impossible. This motherboard, like the HDD, is of a very budgetary level, but there is no money, and I had to sculpt from what was.
PPS Shadow copies and file deduplication are enabled on the server, during measurements everything except shadow copies was disabled (they will be deleted), users on the server did not work, the load was zero.
Screenshots:
Raid array

Plain disk