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Delay in Compositor on frame change
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System Information
Operating system: Linux-4.4.0-151-generic-x86_64-with-debian-stretch-sid 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.26

Blender Version
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-26 08:15, hash: rB250995d67b77
Worked: 2.79

Short description of error
In Blender 2.79 I could cycles through an image sequence with realtime feedback in the compositor, now there is a significant delay before the new image is displayed.

Exact steps for others to reproduce the error
Open the archive below. There are 20 frames from Tears of Steel and 2 blendfiles, one for 2.79, one for 2.80.
Open the one for 2.79 and cycles through the frames with arrow keys. The content of the backdrop changes almost immediately.
Now open the file for 2.80 and cycle through the frames: You'll notice a significant lag. It's as if the compositor is waiting for something until it does something.



Event Timeline

Actually, it seems the problem only exists when using a MovieClip as input. When trying the same clip as Image Sequence, performance is back to normal, with realtime feedback.
Smells like teen depsgraph!
So maybe something for you, @Sergey Sharybin (sergey)? :)

This is not dependency graph.

Weird it sounds, this is same root of the issue as T66872: compositor will use own caching system for the movie clip, and that system is created and tossed away on every re-compo. This is also why old-style color management tagging is not properly refreshed in T66872.