As for the headers, they are usually just called USB 2.0 or USB 3.0 headers. So i take it you mean the USB 3.0 header, where the front USB 3.0 connector goes on:
USB ports on the case not working, or only working very slowly, can often be a problem with the pins on the USB header of the board. Those pins on the USB 3.0 header are extremely fragile and thin compared to the USB 2.0 headers, and the plug doesn't guide them very well. In my opinion, the USB 3.0 headers are a bad design. When you force the plug on there with the pins not perfectly lining up, you can easily bend a couple without noticing. Then when you unplug it to check, and try to bend them straight, they tend to break right off. See here for example,
https://superuser.com/questions/166...-3-0-on-this-motherboard-if-it-has-broken-pin
This even happened to myself once, and i build PCs for people all the time. I saw that i bent one of the pins, tried to fix it, and it broke off immediately. Once this happens, then depending on which pin broke, you may not get a USB 3.0 connection anymore, it will be stuck at USB 2.0 speeds with are about 35 MB/s tops.
The actual write speed though depends only partly on the port's capabilities. Often times, the attached storage device is the limiting factor. So for example, when you put a mediocre USB stick on there with maybe 20 MB/s max write speed, it doesn't matter what the port can do. On your board, you only have 5 Gbit/s ports anyway, so the normal "USB 3.2 Gen 1" previously known as "USB 3.0" ones, with a theoretical maximum speed of around 500 MB/s, but usually below. If you were to max out the port's capabilities, say with a very fast external SSD that doesn't throttle after some time, then transferring 100 GB would take under five minutes. BTW, GB usually means GigaByte and Gb usually means Gigabit (i saw you writing Gb when you probably mean GB).
Split off? Are you sure? That makes no sense, you always have two USB 2.0 headers, even on a lower-end board model. These are your board's USB headers:
View attachment 173469
What model is your case?