1) Could it be Raspberry pi 4 runs too hot and starts to throttle?
2) Is your file system ext4?
3) Try 1 hdd first if you have not, maybe not enough power?
Thanks for your reply.
1. No, I am using Heatsink and Fan and the temperature is always below 46 degree celsius.
2. Yes, One of the Hard Disk is EXT4 and another is NTFS.
3. I tried shuffling them, using with and without power hub but no luck.
Also I am getting 120+ MBps speed on these hdds with Rpi 4 but with OMV 4, this slow speed issue is with OMV 5 only.