High ping on one client? I read the specs and that hardware is questionable. How old is the HDD? average life expectancy is 2-5 years. If the hardware in the HDD is getting old, it can cause some bad issues. I have personal experience with this. I have an old WD HDD I keep for random things as it is old and doesn't spin as fast anymore due to age. The newer one (same model and specs, just newer) maxes as 200 MB/s while the older one cannot go higher than 25. I suggest checking the R/W speed of your HDD and see if it is on par to it's original specs. Below is a few useful links.
https://www.raymond.cc/blog/measure-act ... r-windows/
http://www.buildcomputers.net/hard-drive-benchmark.html