When I built the new machine it booted into Linux Mint that had been installed on the original I7-3930K. It ran fine and was crunching CSG WU in 10400s. I put a new SSD drive in the machine and did a "clean" Linux install. The latest Mint Xcfe is 18 Beta. When the installation was complete I started crunching CSG again and the WU were taking 14500s ... 40% longer. I was after hours so I let it run for 24 hours and nothing changed.

After a day I did a clean install of Mint 17.1 (Xcfe) and the WU time returned to 10400s.

Assuming nothing went wrong with my original Mint 18 Beta install (who knows) then it appears that it may not be ready for prime time yet ... maybe that's why they are calling it BETA

Your mileage may vary, but I'm quite content w/ Mint 17.1 --- at least for now.