It is hard to say as multiple things can affect performance and transfer rates.
- An app is accessing one of the drives in addition to Finder transferring the data thereby slowing down transfers as both processes must compete to access the drive
- File system corruption on source drive
- Fragmented files on source drive...must read file data from multiple tracks on the drive...all take time to move the drive head
- One hard drive may just be slower, or may be wearing out, or may even be starting to fail.
- Other devices connected to the USB hub may be interfering with the transfer
- USB older transfer protocol does a block transfer which is inefficient & problematic.
You can run Disk Utility First Aid on each of the drives to check for possible file system errors. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then you will want to erase the drive and restore from a backup.
You can attempt to check the health of the external drives by using SMARTReporter (normally I recommend using DriveDx, but their download button/link has been broken for a couple weeks now). You will need to install a special USB driver as well in order to attempt to access the external drive's health information. Unfortunately some external drives (adapters, docks, and hubs) may not allow the necessary communication even with the special USB driver.
Instructions for accessing the drive's health information using SMARTReporter:
Within SMARTReporter click on the "Drive Checks" icon at the top of the app window, then make sure "S.M.A.R.T." tab is highlighted, then click "Advanced Tools" tab in the middle of the app window. Then click the "Show Attributes" button for the drive you want to inspect....copy, the health report/attributes and paste the contents of that window here using the "Additional Text" icon which looks like a piece of paper. I will be happy to examine and interpret any reports that are provided.
Drive health monitoring apps like SMARTReporter and DriveDx are usually good at analyzing and alerting to hard drive failures, but any "Warnings" or "Failing" alerts regarding SSDs is more of a heads up thing to tell you to manually examine & interpret the SSD's health information since these alerts do not always mean there is a problem (or at least not an immediate problem, but you do need to keep an eye on any future health attribute changes). Interpreting SSD health is not easy especially with the newer NVMe SSDs which typically provide very little health information.
It is best to make sure your external drives are USB3.x and support the UASP protocol, but each device in the chain between the Mac and the drive must also support UASP. Also, UASP prevents other USB devices (especially older USB1/2) from interfering with USB3.x UASP device transfers.
https://www.startech.com/en-us/blog/all-you-need-to-know-about-uasp
