With the same exact test file I posted here, it takes 2 1/2 times as long to render with MLT 18 -but it is much higher quality - and no black fireflies ? Also MLT 18 shows way more of the Translucent BSDF that I slightly mixed into the prism, to give it some whitish glow, so I brought down the " fac " from 0.05 to 0.03 in the mix shader - and left it there in all of these renders so you can see the difference .

rendered scene above at 4 times the samples, was at 50 but once it gets to about 70 passes it starts showing "black fireflies" - even tried it on "Suzanne" with the same spectral node AND lots of ambient light - same problem?

MLT 19 (holyenigma MingW build) really good fast CPU renders (once I messed with it a little, and followed Lucas' suggestions) on a "physically" simple scene, with very complex "real" spectral/caustic lighting, using MLT 19 holyenigma MingW
A useable "draft" in Around 10 minutes 1920x1080 50 passes, adaptive sampling at 2.5.

@Thomas Dinges (dingto) Please accept my sincere apologies, from now on I will keep my input here only relative to "beta testing/suggestions" of the code in development - if thats appropriate? I won't waste your time via your "automatic email" from this (or any developer) thread.
I am very grateful for all you do for the 3D community, and don't want to be disrespectful or wasteful.
I became a bit "over excited" when I read about Lucas' enthusiasm towards moving forward on implementing GPU, MLT and PIS and should have thought first to send my "cheering" to him from an appropriate forum/channel.

Drum rollll !!!! xxxoooxxxxooo :) :) :) * * * Go Lucas! Go * * * Lookin forward -Thanks in advance ;)

I see you upoladed

using the build lukasstockner97 posted Fri, May 2, 10:11 PM

tried it with 96x96 tiles, still crashes on render (13/60) and hangs the CPU drawing almost full CPU power until I force task manager to stop blender.
Thanks again

rendering of "BMW1M_adptiv_sampl.blend" freezes after about 27/150 tiles (64x64), and continues to consume 98% of CPU power - have to "kill" (end task) blender using task manager, takes about 5 seconds to "kill" blender after clicking button, and minute to get to task manager, cuase the PC is responding so slow. After stopping blender the PC runs fine & can try another instance again without any noticeable problems

found out why it wouldn't load - there needs to be a folder named " 2.70" , with only the folders you already included ( datafiles, python, scripts) inside of it , then it opened!

Just replaced the RADEON 6450 with a nVidea 550ti, and this build still won't load in my particular system.

Your build
http://www.mediafire.com/download/1lkr2mqdd521atu/Blender_270_Metro_VC2013_x64.zip
won't load on my Win 7 PC, it crashes before ever loading.