4k Video Editing in iMovie

[AlexGraphicD]

I own a late 27' 2015 iMac, M395 3.3 GHz i5 8 GB RAM 2TB Fusion, and I am doing some casual editing of 4k footage in iMovie but I am experiencing lots of slow downs and lag when playing the footage on the preview screen and editing. My question is, would an upgrade in RAM solve this issue or is it that the processors on the 2015 Macs can't handle 4k smoothly no matter how much RAM available?

I know that the 2017 updated models have no trouble with 4k. I just don't feel like spending a fortune on a new upgrade if the RAM upgrade would solve the issue.

Also, when trying to export on youtube a 4k video, when it uploads, youtube downsizes the video to 1080p.
I assume that iMovie makes all the necessary changes to the bitrate and codecs so that it is compatible with youtube, so I am not sure what is going wrong here that youtube would downsize the quality.

 

[joema2]

I own a late 27' 2015 iMac, M395 3.3 GHz i5 8 GB RAM 2TB Fusion, and I am doing some casual editing of 4k footage in iMovie but I am experiencing lots of slow downs and lag when playing the footage on the preview screen and editing. My question is, would an upgrade in RAM solve this issue or is it that the processors on the 2015 Macs can't handle 4k smoothly no matter how much RAM available?..

It's not a RAM issue. You are editing H264 4k which is inherently CPU-intensive. If you've added compute-intensive effects, those are GPU-intensive. iMovie uses a similar rendering engine to FCPX, which is very efficient, but 4k H264 can be challenging for any editing software on any platform.

There are differences between various 4k H264 codecs. E.g, the variant used in Sony's XAVC-S is quite sluggish relative to similarly-encoded material at the same bit rate and chroma sampling from Panasonic's GH4, GH5 and DVX200. In general DJI and GoPro material is laggy.

On your current hardware, your options include getting FCPX which support proxies. Once generated that makes everything very fast. Likewise DaVinci Resolve supports proxies and there's a free version.

If it's slow due to compute-intensive effects, defer adding those until late in the edit. First do all your content selection, clip ordering, clip trimming, etc. Only then add effects. This avoids burdening the timeline when you're making rapid changes.

4k is literally 4x the data of 1080p, and your computer didn't magically get 4x faster overnight. At least iMovie (like FCPX) uses Intel's Quick Sync which accelerates H264 encode/decode. Without this it would be much slower.

The Kaby Lake CPU in the 2017 i7 iMac has improved Quick Sync, and on FCPX it's about 2x faster than the 2015 i7 iMac for H264 encode/decode and playback. I assume that's also true for iMovie. However even the 2017 can be sluggish on multi-camera 4k H264, depending on what codec version. I think this also applies to the 2015 vs 2017 i5 iMacs but I haven't tested those.

Resolve also uses Quick Sync but Adobe Premiere 2019 only uses it for output or encoding to a file, so 4k H264 playback is more sluggish on that software (at least for Macs).