The way I would view things here is that this:
and
and
indicates a broken setup. If you want to call it a “bottleneck” feel free. But it is broken.
lspci is the typical tool used. I won’t be able to give a tutorial here, but you can find various writeups, including responses I have made on these forums. the -vvv switch and the switch that selects tree output are relevant here. lspci has a man page as well as command-line help.
You’ve made a few statements here such as “I do not have much background on this” that lead me to suggest that you may be better off 1. updating the system bios on your motherboard and 2. take this issue up with the motherboard vendor.
Even if you educate yourself and learn how to make changes to these PCI settings, they will not persist through a reboot, and there is nothing you can do to change that, other than repeating your changes every time the system boots. If you need changes to these settings, the right way is via the system BIOS.