Rendering is very slow on a new iMac with 40GB of RAM

I've got a fairly long video, which runs for about 1 hr 25 minutes in 1080p and 60fps.

I have added a Text+ node with fusion with a timecode.

When the rendering first started, it was rendering faster than realtime - was due to finish in under an hour.

As time goes on, the rendering gets exponentially slower.

The rendering is currently 12% complete and it has an estimated 5 hours 22 minutes of rendering (and rising!) to go.

If I'm understanding the UI correctly, I think it's rendering at 7fps.

I first tried with version 15 and when that seemingly stopped at around 5% I upgraded to the latest 16 beta.

I find it difficult to accept that this performance is normal given the specs of the machine I'm rendering on:

Screenshot 2019-08-05 at 14.58.09.png (34.72 KiB) Viewed 7213 times

I'm rendering to MP4 with the default YouTube 1080p settings (the source video is H.264) ffmpeg output below:

Code: Select all

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'vid.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.20.100
  Duration: 01:24:42.13, start: 0.000000, bitrate: 11870 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv), 1920x1080 [SAR 1:1 DAR 16:9], 11731 kb/s, 60 fps, 60 tbr, 16k tbn, 120 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 124 kb/s (default)
    Metadata:
      handler_name    : SoundHandler

It's now 5fps and 7 hours (and rising).

Sounds rather too slow.
Try forcing in Resolve main preferences for GPU Metal or OpenCL (from default Auto).
Another issue may be that your iMac is getting overheated and throttling heavily your already weak CPU.
It still should be faster just for HD export. Maybe it's Fusion node (try disabling for a test).
What is your source type?
RAM pays not that big role. You either have enough or not. Once there is enough then it doesn't matter if you have 32GB or 128GB or 1TB. For HD projects 16GB is plenty enough (assuming only Resolve is using it).

It seems my GPU supports either Metal or OpenCL and as far as I can understand OpenCL performance is usually better.

I tried both (rather than Auto).

Unfortunately I cannot see much difference.

Perhaps I should try rendering in Windows via Bootcamp, instead?

So, I did a bit more testing and indeed the Fusion node does impact performance.

If I render without any Fusion nodes at all, it does render much quicker, though it was still showing signs of slowing down, just at a much slower pace.

In case it matters, the Text+ node is of TimeCode type. It has a global in / out set so it only shows for part of the video (which is most of it) there is an offset so it starts from zero when it comes in. It's using a particular font called Bank Gothic, and it is white filled text with a black outline.

I'm not sure if performance is expected to be impacted that greatly or not, or whether I can do something different.

Good suggestions. I also use a terabyte of internal SSD on my iMac 27” late 2015. And I render to the fast internal SSD with all media on a Pegasus RAID 10. My next computer will have at least 2 TB internal SSD but in 2015/16, that wasn’t an option).

I always set the render speed to 2 or 5 or 10 fps subject to the number of effects or stabilization or noise reduction or ambient air temperature as throttling is always my render performance limiter. All I do to deal with summer temperatures: leave doors and windows open and have a larger fan blowing across the back of the iMac.

Apple should be sued for the ludicrous design of the back including the tiny rear vents under the arm of the stand that traps hot air—these designers never test computers in real world situations but must have air conditioned rooms set below 20 degrees Celsius with huge air exchange rates.


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They are violating the most important rule in design: "form follows function".
Not the other way around.

Can you use the Timecode burn-in feature to avoid the Text+ slowness? Or are you doing something special with the timecode display?

They are violating the most important rule in design: "form follows function".
Not the other way around.

Steve Jobs always preferred form over function.

Mr J Ive definitely was obsessed with form and he had a major influence on Steve J. Both out of the picture and the cheese grater is back! Stand definitely better design and only $1,000 USD.


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I agree with Peter. If you are adding the timecode for your own use (ie it is not intended to be part of the final product) or if you are not too worried about the look, you can go to Workspace->Burn-in and set the source or record TC and put it where you want it in the video. When you deliver, be sure to select "same as project" in the Data burn-in under Advanced Settings.

If you want a specific style of text to appear in the final product, you might want to try some other means. I found Text+ from fusion to be really slow. I have a project that I was using that with and it was unbearable even on my iMac Pro. There are two things that sped it up: using proxy mode at 1/2 resolution helped a bit; using 3D text made it much faster.

Also, since your project is 60fps and you probably don't need that many frames of animation, you could raise the Proxy ratio in the General tab under the Fusion menu. If I understand this correctly, these ratios are the number of frames of video for every frame that gets rendered when Proxy is selected or AutoProxy is on. If you set it to 10:1, you will still get 6 frames per second.

Thanks everyone for their replies.

I wanted to follow up with my findings. I really liked DaVinci Resolve but my new video editing journey will be continuing without it.

For completeness, I wanted to make sure I tested rendering the video on Windows (on the iMac) and I sadly got the same result, though I optimistically decided to leave it running.

Unfortunately, after 12 hours, it hadn't even completed 50% of the rendering and had slowed to around 3fps rendering.

Indeed I do want 60fps because the video was gameplay footage which runs at 60fps and wanted to make use of 1080p 60fps on YouTube.

The burnt in timecode options weren't suitable as I did need to start the timer at a specific time in the video (from zero) and I wanted it to have a particular font.

In the end I found another video editing software VSDC which is certainly less feature rich than DaVinci, but does have the timer functionality nailed in a useful and simple way.

More importantly, at the same rendering quality as DaVinci (typical settings for YouTube, 1080p, 60fps etc) it was able to burn through the rendering in just over an hour (so, slightly faster than realtime).

Screenshot 2019-08-08 at 12.01.00.png (16.81 KiB) Viewed 7009 times

If any DaVinci developers / staff come across this, I don't believe it should be the case that Resolve should take so long. This may well indicate a significant bug because, seemingly, it is possible to do what I wanted in a significantly more acceptable period of time.

Good luck, and thanks all again for the replies.

i don't think it's Resolve, it's the underpowered CPU and thermal throttling

my machine with 2x 12c xeon's / 128g ram / 2x 1080Ti's / 64Tb (16x4Tb) raid on 12g SAS / cool server room / conditioned power - renders Dc2k anything at around 200+ Fps all day long - Fusion included, no slowdown, no fans cycleing up, no drama.. just rok solid

it's not the software, it's the machine... bring a solid server class machine to the party and the same software is gonna fly

Sorry, how can it be the machine if the other software can produce the same result on the same hardware in a tiny fraction of the time?

[Peter Chamberlain]

Different software has different configuration requirements. For that iMac you should definitely use Metal.
And it would be good to explore more details on this project so we can improve.

But good that you found something that worked In the meantime.
Depending on what you are doing in Resolve, and the other apps features, you can edit and grade in Resolve, render without the Fusion effect to an intermediary format and then use the other app for the timecode overlay and render to h.264.

I’m happy to review the Resolve drp if you export it from the project manager.

Hey Peter,

Yeah I tried three times on macOS using Metal, OpenCL and Auto (I assume Auto would map to Metal anyway).

Also tried on Windows (same hardware) with OpenCL and Auto.

All options exhibited the same poor performance.

It's worth noting that an earlier test which was a straight up render with no Fusion did seem quicker though was showing signs of slowing down, just not as much.

I'll try and get the drp file to you.

[Peter Chamberlain]

hi, the QA guys are reviewing but in the meantime, using the data burn in I can get a similar TC field at the top of the image.
This avoids the need for the Fusion text+.

Thanks Peter,

Can you start it at a specific time, and run it for a specific duration?

The purpose is to display a timer for part of the video.

I think the main problem here is rendering the timeline directly to the h264 longGOP file.

If you render the timeline to any intra-frame codec first, such as ProRes or even uncompressed file, the render would have been much faster. And compressing that master rendered file to h264 either outside or inside Resolve would also have been much faster as it does not have to deal with any Fusion node processing. This two step process sounds cumbersome, but is actually faster, gives you much more control over the whole process, and can give you higher quality result in the end.

Rendering the timeline full of LongGOP material and fusion nodes using multipass LongGOP encoder is just exercise in masochism : )

I think the issue is Fusion too heavy gun for such tiny things like small texts/counters.

Thanks Peter,

Can you start it at a specific time, and run it for a specific duration?

The purpose is to display a timer for part of the video.

No you can't. What we do here is take the specific portion that would need the data burn into a separate timeline, put the data burn there and export the portion.
Then we patch the portion in the main timeline and render the full timeline out (without any more data burn).
It may sound a bit convoluted, but it's a workaround and it's fast.