Starting with Linux Kernel 4.7, the Radeon R9 290 started showing very low
performance, about 20/30% of what is available up to kernel 4.6. As can be seen
in this article, the performance regression started even before the launch of
kernel 4.7 RC1, using Alex Deucher's drm-next-4.7 branch:

http://www.phoronix.com/scan.php?page=article&item=radeon-drm-next-47&num=1

Some time later it was speculated that the problem may be with DPM:

http://www.phoronix.com/scan.php?page=news_item&px=Linux-4.7-R9-290-Regression

To replicate the bug, install Ubuntu 16.04 64bits and install kernel 4.7 from
here (preferably above rc5):

http://kernel.ubuntu.com/~kernel-ppa/mainline/ 

The performance regression will show right away. Rebooting the system and
choosing a kernel version up to 4.6 will fix the bug.