Choppy 6K H265 on PC but smooth on laptop

I'm running Davinci Resolve Studio 18.6 on my PC and laptop.

PC
i7 10700K
RTX 3070
32GB RAM
4K monitor + 1080 monitor
SSD

Laptop
i5 12500H
RTX 3050 (Laptop obviously)
24GB RAM
1080 monitor
SSD

The file is from a Panasonic S5ii, 6K footage.


On my laptop, playback is smooth and simple editing, a LUT, etc has no problems.

On my PC, it is choppy playing back at about 15fps. The GPU is at ~20%. (When I edit 4K, it is smooth and is also at ~20% GPU).


Same Davinci Resolve settings on both:
GPU - Auto off, CUDA
GPU selection - Auto off, selected the GPU
Decode H264/H265 using hardware acceleration - NVIDIA selected

Graphics drivers updated


Any ideas why my PC isn't playing back as well as my laptop (which nominally has lower specs)?

I've tried various combinations of the following with no impact:
- one or both monitors running
- tapping my head and rubbing my tummy
- setting 4K monitor resolution to 1080
- changing project and timeline settings to lower (1080) and higher (Ultra HD) resolutions and various permutations
- putting the file on HDD, SSD, external HDD

The laptop is smooth even when Project and Timeline resolution are set to Ultra HD.

Just for kicks, I also turned off GPU decoding and it was the same. And 4K playback was smooth with GPU decode off. Which makes me wonder if Resolve is just not using the GPU ever?


(Yes, I know proxies are better. If I make proxies it works fine on the PC. I'm just puzzled as to why it works smoothly on my laptop and not my PC. If I can get it to run smoothly on my PC I can avoid proxies, especially because I'm not doing heavy grading or Fusion much.)


Sorry if there is another thread covering this but I can't quite find the same issue. Thanks in advance for any help.

Could it be that the laptop has the right hardware decoder for that specific ’flavor’ of H.265 and the desktop doesn’t?
Check Intel's data sheets for your CPU's QuickSync.

Could it be that the laptop has the right hardware decoder for that specific ’flavor’ of H.265 and the desktop doesn’t?
Check Intel's data sheets for your CPU's QuickSync.

That’s what I was thinking too. The laptop cpu must have QuickSync which has a better H.26x decoder. An i7 12700 or higher would have QS and would do hardware decode like the laptop.

Thanks for your tips.

So a couple of questions if it's not too much to ask anyone who knows. My Googling skills seem to have hit a limit.

1. My PC (10700K, 10th Gen Comet Lake) according to Wikipedia can decode H265 4:2:0 (which is what the 6K file is).

But later generations add (assumedly) faster implementations. The laptop being a 12500H (12th Gen Alder Lake) is newer.

Would it be safe to assume this is the main difference?


2. Does the CPU (and its ability to decode H264/5) affect the GPU decoding? As in, what difference does selecting GPU decoding for H264/5 make? Or is it that it uses both and the GPU helps?

(Because my assumption was that GPU decoding meant that the decoding would all (?) be pushed to the GPU. But I guess it's dangerous to assume too much.)


Sorry if these seem rookie type questions.