What on earth is going on with my RENDERING?

Totally bemused. I have the same file, the same save, when i render it on a development build e.g. one compiled on the 15th Jan, it took 1 min 15 seconds, it used 100% of GPU and CPU. GTX 1070 & Ryzen 1700X.
Loaded up the exact same save on the official Blender 2.79 with GPU mode selected and its at 48 mins so far and says another 48 mins remaining!! I totally realize that the official version is not implementing this hybrid CPU +GPU, but i was expecting about a 2-3 min render not 2 hrs. (16x16 tile size btw).

Also if anyone could shed any light on it, i ran a large resolution render last night with the development build and it got to about 90% in around 2-3 hrs but then sort of went to sleep and plodded along (almost ground to a halt) for 8 hrs just to finished the last 10% which was only white diffuse background anyway, then when i got up and came back to my PC it sort of woke up and got on with finishing it at proper speed again…

Here are the screen shots to give you a perspective.


You are mixing things up.

As a rule of thumb you should use big tiles for GPU render (like for example 256x256 or 512x512)
and small tile sizes when rendering with the CPU (16 x 16 or 32 x 32)

Only with the newer builds you can render with small tile sizes on GPU + CPU, but maybe this change in the future
and you have to render with small tile sizes.

For your card I recommend a tile size of about 480 x 270, this gives you the best result in general.
For your render I would render it at your full resolution you have: 442 x 537, this could be the fastest for you.

That must be it. I was thinking that CUDA did not need big tile sizes anymore because of the 16x16 tile size i use in the hybrid CPU+GPU… gargantuan difference it makes. 96 minute render down to 2 mins. I hope the hybrid rendering gets implemented officially soon, these sorts of software optimizations do far more than buying newer hardware ever could, and along with the denoiser, it obliterates the old render times of my old core i5 ivybridge setup.

BTW one of the reasons i rendered on the official build, is that the development build was screwing up the UV mapping and at one point screwed up some of the geometry (something to do with displacement). So they are definitely not bug free builds of blender.