Specifics please.
Underperforming so much: according to whom? Your 2666MHz ram is at 2400MHz because either the xmp is not enabled or you are chipset limited on that motherboard, so naturally it's going to come across as slow compared to other ppl using 2666MHz ram at 2666MHz. So also chose to go with a 1x8Gb stick, so are losing out on any possibility of performance gains for dual channel. Also 8Gb is pretty much minimum for Windows 10, which can use @ 2Gb itself, so any games running at 5.5Gb+ can be pagefiled to the ssd/hdd which hugely slows down fps.
Looks bad. Well that's usually an indication of not having the correct settings for resolution or trying to force the wrong frequency out of the monitor.
As to the storage, I'm guessing that when you installed/reinstalled windows, you did Not first unplug the data cables to the hdds. Windows likes to library everything under 1 assumed umbrella, so if there's multiple drives, it'll assume that All drives are no different than partitions on a single C drive. Meaning both your hdds are treated the same as the ssd, it'll constantly search, update, index, the hdds every time. They aren't treated as storage, but part of the OS drive. This can and will put gaming performance in the toilet.
I'd suggest you unplug the data, and do a clean install on just the ssd hooked up. Keep the hdds as a seperate drive, not included with the OS.