Blast Rain of Trarthus FPS drop

whenever I use the skill the framerate (fps) drops from 60 to 30, the the white ms-line goes from 17 to 30+ and the gpu-line from 10 to 20+.

I've tried it in an empty instance, where at first all the diagnostic lines were even. As soon as I used Blast rain the lines went up.

Tried the same with Sunder of Trarthus but that skill has no issues, no spikes.