I'm wondering, that since Jumbo Frames was supposed to be better for bulk transfers, why am I seeing these results? Is it the ElRepo drivers I used to enable higher MTUs or possibly some kind of oddity with the realtek NICs I am using? Or am I mistaken about the benefits of jumbo frames and that they are only beneficial in specific configurations, perhaps only with higher network speeds and enough packets at 1500 MTU to overwhelm packet handling hardware?
In short, the entire path from origin to destination must be configured to support jumbo frames. If not, then the devices at various points in the path will attempt to scale and you'll end up getting fragmentation and higher overhead in the devices leading to worse overall performance.
Have you verified this is the case?
Of course, the quality of the driver for your NIC will also play a role.