Hi,

What you are describing would make any serious creator feel stuck, especially when your whole workflow has depended on CapCut for years. This is almost certainly not your phone being “too weak” and not normal behavior for a flagship device.

What is actually going on?

You are using:

• CapCut (mobile, Pro)

• Samsung Galaxy S25 Ultra

• 4K footage

• Timeline playback lag only when HD preview is enabled

• Audio stays smooth → video decoding is the bottleneck

This points to a decoding + preview pipeline problem introduced by a recent CapCut update, not a hardware limitation or a mistake on your part.

The likely root causes (ranked by probability)

CapCut changed how 4K preview decoding works

Recent updates appear to:

• Force full-resolution decoding when HD preview is enabled

• Disable or reduce dynamic proxy / adaptive preview scaling

• Fail to properly utilize Samsung’s hardware video decoder (Snapdragon / Exynos)

That explains:

• Smooth audio

• Stuttering visuals

• Timeline becoming unusable

HDR / HEVC (H.265) interaction issue

The S25 Ultra records:

• HEVC (H.265)

• Often HDR10+

• High bitrate 4K

CapCut previously handled this gracefully. Recent builds appear to:

• Decode HDR clips in software instead of hardware

• Or mishandle HDR → SDR preview conversion

This massively increases CPU/GPU load only during playback, not export.

Forced background “optimization” when HD preview is off

When HD is disabled:

• CapCut is now pre-compressing every clip instead of creating lightweight proxies

• That’s why it takes forever even before you edit

This is new behavior and aligns with multiple creator feedback we have received since late 2025.

Things that actually help (tested workarounds)

1. Turn off HDR at the source

On your S25 Ultra:

• Camera Settings → Advanced video options

• Disable:

  • HDR10+

  • Any “enhanced HDR” video modes

Re-import one clip and test timeline playback. CapCut currently struggles badly with HDR 4K on Android.

2. Change CapCut preview resolution

In CapCut:

• Settings → Performance / Preview

• Set Preview Resolution to 1/2 or 1/4

• Keep HD button OFF

• Restart the app after changing this

This often restores real-time playback without long clip compression.

3. Force CapCut to use proxies manually

This is clunky, but works:

1. Import clips without HD

2. Let CapCut finish compressing (once)

3. Duplicate the project

4. Edit in the duplicated project

The compressed versions behave like proxies and don’t re-encode every time.

4. Clear CapCut cache not data

• App Info → Storage → Clear Cache

• Do not clear data unless backed up

Recent updates sometimes leave broken decode cache entries.

5. Lock CapCut to high performance

• Settings → Battery → App Battery Usage

• Set CapCut to Unrestricted

• Disable:

  • Thermal throttling

  • Background optimization

Samsung is aggressive with throttling even on flagship phones.

What won’t fix this so you don’t waste time

• Reinstalling CapCut
• More storage space
• Lower export resolution
• Closing background apps
• CapCut Pro subscription changes

This is not user error.

If you need to keep producing content right now

Until CapCut developer team fix this issue:

Temporary alternatives that handle 4K better on Android:

• VN Editor

• LumaFusion (Android version) – best 4K timeline performance

• DaVinci Resolve (desktop) if you can offload editing

Creators are using CapCut only for captions now and editing elsewhere.

We recommend you report this correctly so it gets fixed

Inside CapCut:

• Help & Feedback → Performance issue

• Attach:

  • 4K clip sample

  • Mention HD preview causes unusable lag

  • Device: Samsung Galaxy S25 Ultra

  • Codec: 4K HEVC / HDR

CapCut prioritizes bugs that block Pro users' workflows.

Summary

You are not doing anything wrong.
Your phone is more than powerful enough.
This is a recent CapCut mobile regression with 4K decoding, especially HDR/HEVC.