I just looked at your first post. Slowdowns to me can be related to a hard drive going bad. I found the model ssd in your system report and then went looking.
I found an interesting thread about the model you have, PM851,
Solved: PM851 SSD Slowdown (Like SSung's 840EVO)?? - Dell Community
Seems like it slows down after about a month of use.
Then the last post mentions the last BIOS update (I saw where yours is from Feb. 2018) might contribute to the slowdown.
Hardware is not my strong suit so wait until someone with more experience gives some thoughts.