A colleague of mine has a similar Samsung SSD disk but didn't encounter any performance issue after installing v8. Does renaming C:\Windows\System32\drivers\ehdrv.sys and C:\Windows\System32\drivers\eamonm.sys in safe mode, one at a time, make a difference?
Well, I removed the version 7 and did the test again with version 8, the same problem occurs.
One possible solution was disable Scan on File open and File creation (Image on the link below)
hxxp://i.imgur.com/k1xIp9u.png
In this way I have no loss of performance, and it works as if I had disabled the antivirus.