There are several possible causes for this:
- If you triggered "index build" for some reasons, it could take a while to rescan all storage files.
- If you have a partially damaged disk, the damaged point may stop the software from working correctly.
Hard disks are known to wear-out after prolonged usage. Building an NVR server with old disks is particularly risky because old disks may contains bad-sectors that may lead to data corruption. Some disks are equipped with self-repair function that automatically moves data from bad-sectors to good-sectors so it does not show damages in an obvious way, but the self-repairing mechanism takes time, and therefore reduces the overall bandwidth. The read-write-heads of a damaged disk could also cause the disk to become much slower.
To reduce this risk, you should periodically engage maintenance to your NVR system by measuring disk performances. If the performance is degraded since last measurement, it could mean a danger signal that the disk is about to be irreversibly damaged.