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