Apple updates iMovie for Mac with HEVC support
Following the release of maOS 10.13 High Sierra on Monday, Apple has updated its consumer level video editing tool, iMovie, with support for High Efficiency Video Encoding, a tentpole video feature introduced alongside the company's latest hardware and software releases.

The latest iMovie for Mac brings the editing app up to speed with macOS High Sierra and iOS 11, both of which rely on HEVC to compress ultra high-resolution video into manageable chunks.
Also referred to as the H.265 codec, HEVC is an improvement over the current H.264 standard that has long been Apple's go-to for efficient video encoding on both iOS and macOS. The new version includes better motion compensation for fast-moving scenes, larger computational errors for difference-coding, and other improvements that allow HEVC to compress video down to nearly half the size of an H.264 clip.
With today's update, users can import HEVC files like 4K content captured on iPhone 8 directly into iMovie for cutting. Prior to the app update, such files were available for viewing in on Macs running sixth-generation Intel Core processor or newer, but editing tools were unavailable.
Still missing, however, is HEVC support in Apple's professional-level apps, including Final Cut Pro, Motion and Compressor. For now, pro film makers have to import and convert HEVC H.265 video into the Apple ProRes format.
iMovie for Mac is available as a free download from the Mac App Store.

The latest iMovie for Mac brings the editing app up to speed with macOS High Sierra and iOS 11, both of which rely on HEVC to compress ultra high-resolution video into manageable chunks.
Also referred to as the H.265 codec, HEVC is an improvement over the current H.264 standard that has long been Apple's go-to for efficient video encoding on both iOS and macOS. The new version includes better motion compensation for fast-moving scenes, larger computational errors for difference-coding, and other improvements that allow HEVC to compress video down to nearly half the size of an H.264 clip.
With today's update, users can import HEVC files like 4K content captured on iPhone 8 directly into iMovie for cutting. Prior to the app update, such files were available for viewing in on Macs running sixth-generation Intel Core processor or newer, but editing tools were unavailable.
Still missing, however, is HEVC support in Apple's professional-level apps, including Final Cut Pro, Motion and Compressor. For now, pro film makers have to import and convert HEVC H.265 video into the Apple ProRes format.
iMovie for Mac is available as a free download from the Mac App Store.
Comments
https://en.wikipedia.org/wiki/High_Efficiency_Video_Coding#Versions
One of the additions was Screen Content Coding:
http://www.streamingmedia.com/Articles/Editorial/Featured-Articles/Screen-Content-Coding-(SCC)-Offers-Improved-Quality-and-Efficiency-for-HEVC-Screen-Captured-Content-111356.aspx
"For the type of content that SCC targets, it achieves roughly an additional 50% of rate reduction at the same quality compared to earlier versions of HEVC, and roughly 80% of rate reduction at the same quality compared to the widely used H.264/AVC standard. In other words, you only need to stream one-fifth of the data using SCC that you would otherwise be streaming using H.264/AVC to achieve the same quality."
This is for screen capture videos including recording gameplay and app usage (tutorials, marketing etc).
There were also licensing issues with the codec so that will have contributed some delay too.