Tested with windows 7 64bit and windows 10 64bit.
CPU ryzen 5 1600
GPU GTX 780
RAM 8GB GDDR4
(bug appears on other computers too)
tested with 2.79 and 2.8
Short description of error
If I rotate object for about 45 degrees around Z axis render time increases more than 10 times.
But in preview mode there is no difference in render time before and after rotation.
Exact steps for others to reproduce the error
just rotate the object on attached blend file around Z axis about 45 degrees and compare render times before and after.
10seconds for 2000 samples before rotation and about 10 minutes after.
Based on a (as simple as possible) attached .blend file with minimum amount of steps