Compatibility Report

• Name of the game with compatibility issues: METAL GEAR SOLID 3: Snake Eater - Master Collection Version
• Steam AppID of the game: 2131650

System Information

• Steam Deck
• Proton version: Proton 11 (Beta)

I confirm:

• ✅ that I haven't found an existing compatibility report for this game.
• ✅ that I have checked whether there are updates for my system available.

Symptoms

While investigating a separate high CPU usage issue in MGS3 under Proton, I noticed that cutscenes can still experience FPS drops even after fixing the original bottleneck. This is exclusively effecting cutscenes and not gameplay.
The original issue was caused by the game busy-waiting between frames using repeated QueryPerformanceCounter calls instead of sleeping. Under Wine/Proton, QPC appears to have significantly higher overhead than native Windows, causing one CPU core to become pegged at 100%.
A simplified version of the game loop looked like this:

while (true){
    if (!16_ms_has_passed()){
        continue;
    }
    process_next_frame();
}

Replacing the busy-wait with a sleep-based approach resolved the excessive CPU usage:

while (true){
    if (!16_ms_has_passed()){
        Sleep(remaining_time);
    }
    process_next_frame();
}

This substantially reduced CPU usage and improved overall frame pacing.
However, cutscene FPS dips still occur even after this fix, and disabling the patch does not eliminate the dips. This suggests the cutscene issue is likely unrelated to the original busy-wait problem.

During cutscene FPS drops, CPU usage is actually low.

This suggests the issue may be related to:

• Proton/Wine scheduling behaviour
• timer/wake latency
• video decode paths
• Vulkan synchronisation stalls
• or handling of the game's .sdt cutscene/video resources
• The issue appears more noticeable under Proton/Linux than Windows.

Reproduction

1. Launch MGS3 Master Collection under Proton
2. Enter cutscenes from Demo Theatre
3. Observe intermittent FPS dips (60-35 FPS)
4. CPU usage remains relatively low during dips (1-2w)