It seems that your Fusion drive (the HDD portion) if failing by the very low read and write speeds:
Write speed: 61 MB/s
Read speed: 77 MB/s
I wholeheartedly agree with Jack-19 that your best option would be to get an external SSD, clone your current drive to it with Carbon Copy Cloner (it might take some time), and boot and run from the external SSD.
If you go that route I recommend you contact Customer Support at OWC (MacSales.com) and get their recommendation of which of their external SSDs would best fit your Mac model, work requirements and budget.
If you go that route and still want additional RAM you can get a couple of 4 GB modules to raise the total to 16 GB from either OWC or Crucial.com. I wouldn't get it from any other vendors.