I have tested almost every renderer in the existence and rendering has been my main focus for past 9 years. Statement that Cycles is significantly slower than other mainstream renderers is factually true. The speed difference is somewhere between twice slower to the order of magnitude slower depending on the scene. This applies for CPU rendering.
For GPU rendering, on a really fast GPU, like GTX1080Ti, the performance of Cycles is somewhat acceptable, but you are limited to quite trivial scenes, otherwise you run out of GPU memory pretty much instantly.
The slowness is actually confirmed even by Tangent Animation in this thread:
Particularly in this post:
The guy states that AVERAGE, not MAXIMAL, but just AVERAGE rendertime per frame for them the was 3.76 hours, nearly 4 hours. For 4k that would be acceptable, but that was for 2k (2048*1080). Average of 3.76 hrs per frame means somewhere between 1 hour and 7 hours per frame for just pretty much fullHD resolutions.
So those shots must be really heavy, lots of translucent foliage, closeups of refractive and SSS surfaces in DoF, etc… right? Well, no… Check the screenshots at the beginning of that thread. The scenes are nice, but completely trivial from the shading point… Exterior scenarios, very friendly for path tracers and mostly flat surfaces with no transmissive properties. These are the kinds of shots that take about 20-30 minutes per frame in FullHD to get to final quality in something like V-Ray or Corona. Hearing 3.76 hours/frame in 2k for such type of shots is horrifying.
On top of that, keep in mind that Tangent has own Blender developer which optimized Cycles significantly, so the Cycles you are using in official branch is even slower.