Slow Transfer Speed Between PCs on Home Network

You still need to test the file transfer speed as advised.

Thanks for these replies. I should add that this problem occurred with relaying an audio stream of 128kbps from one PC to the other which relayed it on. The stream of the receiving PC was buffering and jumping uncontrollably. Usually it's fine 

Using Gigabit ports: transfer a large file (at least 1–5 GB) between two computers on the network and time how long it takes. (I just use copy with File Explorer). Divide the file size (in megabytes) by the time (in seconds) to get the speed in MB/s. The maximum will be around 118 MB/s or 940 Mbps for SSD→SSD transfers but, if it's significantly less, we can investigate.

What is measuring the transfer speed? Some devices like NAS may be indicating MB instead of Mb.

Defective network cables will cause the connections to degrade from 1000 to 10/100 and that makes for a slower transfer.   But the 100 Mbps connections can be seen in the Hub 3 connected devices and on the Windows Network status info.

If you think the internal 10/100/1000 switch of the Hub 3 is the issue then connect the two PCs directly to a 5 or 8 port Gigabit switch & see if it makes any difference when the data transfer does not go via the Hub 3.

Double check the link speed on PC as sometimes the ethernet cable can force it to 100/100 which might explain the slow speed. Sometimes taking it out and re-inserting it can make it go back up to the full duplex speed. The cable itself can be a major factor in whether it goes higher or not too. 

How slow are the 'slow' speeds you are experiencing? 

The Hub 3 is a bad device under all circumstances.  You can google that with the words “Intel Atom” and “Hub 3”.

The Hub 3 is acceptable if you move to modem mode with your own router.  Then you can expect better LAN speeds.