Why is the timeline slow in Premiere Pro?

Question

Video processing

The timeline (preview) freezes very much when playing, rewinding footage. Happens at both Full and 1/4 resolution. Sources in FullHD. This manifests itself without adding effects and other things (just slicing). Save only proxies.
From observations: the processor is loaded at 100%, the video card at 1-5% maximum. GPU acceleration enabled.
Config:
Ryzen 3 1300 PRO
16 Gb RAM
HD 7850
A friend on a much more modest config (G5405, 8 Gb, UHD 610) plays the same footage much better. The video core, accordingly, is also loaded more than the processor.
I installed programs of different years, transferred everything to ssd, changed the video card (to GTX 760) - nothing helped. GPUSniffer also detects the video card.

Answer 1

Because most likely the video is not an editing codec. What kind of mp4 thread do you have.