Rendering speed has become higher in recent builds, but still a bit below the previous speed
Yup, that is what the main Blender Dev team stated in their post. they will tackle the remaining issue in 2.82. 
slightly sad by that but fully understood, as it woudl hold back 2.81 release a bit.
An undefinable amount of time, you mean. So a very good reason, really.
Well, I hope that they will bring optimization to the end, it would be great
Did the tests in all settings, next to no noticable changes on the BMW scene.
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I think the Hilbert Spiral issue only occurs at high resolution images. There the image has to be kept in memory for much longer than using one of the directional methods. The standard BMW scene will not be affected by that. So your problem might be based on something else.
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will test again on my own larger scene at 4k, once the 2.81 is official (thursday 21/11/2019?)
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In the 2.81 official release it keeps making the renders way slower for Opencl.
Like almost double the time.
what scnee did you test? i tested the BMW and though ti didntā return to where it was in 2.8, it was much better then some of the betas that had 2x slowdown.
Iāll retest, and verify
Same here.
The render times with my RX 580 GPU are getting unbearable.
Is this the Nvidia money at work? Seems so.
I am also having the same problem.
Using Radeon VII and 2.81 is slower than previous versions.
Same issue, dual Vega 56, kubuntu 18.04.3, kernel 5.0, driver AMDGPUpro 19.30
2.80, release, 52secs, 100secs one Vega
2.82, old build, 93secs
2.82, latest, 60secs, 113secs one Vega
has already ended the support of the one-year developer from AMD? , Vega 56 used to outperform the GTX 1070 by big margins, now the 1070 is on par with it https://www.phoronix.com/scan.php?page=article&item=blender-281-optix&num=2
well the developer is currently occupied with other thigns, and AMD did sponsor another two developers but more for Vulcan work.
As for the 1070 now being on par, to be honest that is simply cause they put enough development in to get it fasterā¦
Iāll do some more testing with todayās versions and reprot back
So BMW on single Vega 64.
2.80 1m31s
2.81 1m47s
2.82 1m47s (build Dec 12 2019 - fda791ab1241)
NOTE, for 2.82 the compile time isā¦ far far longer then in previous builds. with the first run, render with compile was 2m22s, but second render was inline with 2.81 official.
WIll test dual setup soon
Downloaded 2.81 couple of days ago (Sapphire RX Vega 64 Nitro+) and one of my scenes I use instead of just over 6 mins is over 8 mins. I hope this gets fixed soon.
I have a Radeon Pro Duo Polaris. When I have my CPUs and GPUs checked for hybrid rendering, my two GPU tiles are not showing up in the rendering. Only the CPU tiles are rendeing. Works as expected in 2.80.
Just did a test with 2.81a release. TR1950 + Vega64, i can definitley see both being used, though tiles had to drop to 32x32.
And definitley went down from 1m44s to 1m20sā¦ not a āgood boostā but such is the case with CPUās.
Side Note, nice GPU 
I definitely will test my own scene, but wanted to keep bmw or somethign that the Blender Devās can retest to verify the issue.
so far
2.81 - 2.81a - 2.82(alpha - todayās build) have same resultā¦ 1m44sā¦ but slower then 2.80
Hope they address that in 2.82 full releaseā¦ Else ProRender? :frowning
You might want to test against this bug. This one was using dated NVidia Quadro card at work, but maybe issue also exist on Vega64? I have Vega64 at home, but Iām rendering on CPU only - since Iām a Linux noob I canāt get the AMD drivers to install.
Thanksā¦I did not know there was 2.81a. I will have to get it.