Option 1: add a config file to multithread the process.

Source: https://share.gemini.google/tYeNIu66XREI

1. Go to Steam Installation Folder

Go to ~/.local/share/steam/ or ~/.steam/steam/ on Dolphin

Or ~/.var/steam/ if using flatpak

2. Create a new text file and name it “steam_dev.cfg”

3. In that text folder add this:

unShaderBackgroundProcessingThreads X

unShaderBackgroundProcessingThreads X

Replace X with your CPU's thread count (e.g., 8 for an 8-core/16-thread CPU, or 16 to leave headroom for other apps).

Found your CPU thread using fastfetch or neofetch

4. Save the file and restart steam

Option 2: use Proton GE with ProtonQt-UP

Proton GE has video codecs that help play video in windows game, this video codecs is licensed therefore Steam can’t add it to their normal Proton because it means paying for royalty of those license.

So Steam use other way around by caching those video with the shader which slow down the process.