[UE 5.8.4][Windows][Editor] Editor permanently hangs when a large map is configured as EditorStartupMap before Asset Registry OnFilesLoaded completes

General Issues and Bug Reporting (Unreal Engine)
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Summary

In Unreal Engine 5.8.4 on Windows, setting a large map as EditorStartupMap causes the editor to become permanently unresponsive. The same map opens normally when opened manually after startup. The failing log stops during AssetRegistry OnFilesLoaded.Broadcast. Loading a lightweight bootstrap map first and delaying the target map until Asset Registry initialization completes avoids the hang.

What Type of Bug are you experiencing?

Editor

Steps to Reproduce

  1. Open a Blueprint project in Unreal Engine 5.8.4 containing approximately 24,293 assets and a 186,686,401-byte map.
  2. After the editor is ready, open the target map manually and confirm that it loads normally.
  3. Set EditorStartupMap=/Game/BoonieBears/Map/GouxiongLing.GouxiongLing in DefaultEngine.ini.
  4. Close and relaunch Unreal Editor.
  5. Wait for the startup map and initial Asset Registry scan to finish.
  6. The editor becomes permanently unresponsive during AssetRegistry OnFilesLoaded.Broadcast.

Reproduction rate: 100% in this project. The issue reproduces with both D3D12/SM6 and D3D11/SM5.

Expected Result

The configured startup map should finish loading and Unreal Editor should remain responsive, just as it does when the same map is opened manually after editor startup.

Observed Result

The map finishes loading, Map Check reports 0 errors and 0 warnings, and Total Editor Startup Time is logged. AssetRegistry then logs Starting OnFilesLoaded.Broadcast but never logs completion. The editor remains Not Responding indefinitely, CPU usage drops nearly to zero, and no crash report is generated.

Using a lightweight startup map and opening the target map only after Asset Registry initialization completes prevents the hang.

Affects Versions

5.8

Platform(s)

Windows

For crash reports, include your callstack

Sanitized log excerpts for forum post

Environment

LogInit: Engine Version: 5.8.4-58902052+++UE5+Release-5.8
LogInit: Compatible Engine Version: 5.8.0-55116800+++UE5+Release-5.8
LogInit: OS: Windows 11 (25H2) [10.0.26200.9711], CPU: AMD Ryzen 5 7500F 6-Core Processor, GPU: NVIDIA GeForce RTX 4070
LogMemory: Memory total: Physical=31.6GB (32GB approx) Virtual=60.6GB
LogRHI: Driver Version: 610.88 (internal:32.0.16.1088, unified:610.88)

FAILURE 1 - Direct startup map, D3D12 / SM6

[2026.11.06-06.42.05:250][ 0] Cmd: MAP LOAD FILE=“<PROJECT_ROOT>/Content/BoonieBears/Map/GouxiongLing.umap” TEMPLATE=0 SHOWPROGRESS=1 FEATURELEVEL=4
[2026.11.06-06.42.23:709][ 0] LogEditorServer: Finished looking for orphan Actors (0.001 secs)
[2026.11.06-06.42.24:339][ 0] MapCheck: Map check complete: 0 errors, 0 warnings.
[2026.11.06-06.42.24:373][ 0] LogUnrealEdMisc: Total Editor Startup Time, took 29.005
[2026.11.06-06.42.29:602][ 1] LogMetaSound: Display: Migrated Class Interface paged graph for MetaSound ‘/Game/Hyper/ResourcePack/Audio/MetaSounds/sfx_Random_nl_meta.sfx_Random_nl_meta’
[2026.11.06-06.42.29:758][ 1] LogAssetRegistry: AssetRegistryGather time 0.2391s: AssetDataDiscovery 0.0240s, AssetDataGather 0.0701s, StoreResults 0.1450s. Wall time 28.6640s.
[2026.11.06-06.42.29:799][ 1] LogAssetRegistry: Display: Starting OnFilesLoaded.Broadcast
[2026.11.06-06.42.29:923][ 1] LogMetaSound: Display: Migrated Class Interface paged graph for MetaSound ‘/Game/Hyper/ResourcePack/Audio/MetaSounds/sfx_Weapon_BaseImpact_nl_metaPreset.sfx_Weapon_BaseImpact_nl_metaPreset’
[2026.11.06-06.42.29:927][ 1] LogMetaSound: Display: Migrated Class Interface paged graph for MetaSound ‘/Game/Hyper/ResourcePack/Audio/MetaSounds/sfx_WeaponSwap_nl_meta_Preset.sfx_WeaponSwap_nl_meta_Preset’
[2026.11.06-06.42.31:134][ 1] LogAssetRegistry: Display: Asset registry cache written as 169.3 MiB to <PROJECT_ROOT>/Intermediate/CachedAssetRegistry/CachedAssetRegistry_*.ref

[HANG] Completed OnFilesLoaded.Broadcast is never logged. The editor remains Not Responding indefinitely and CPU usage falls nearly to zero.

FAILURE 2 - Direct startup map, forced D3D11 / SM5

LogInit: Command Line: -NoLiveCoding -d3d11
[2026.11.06-06.49.13:786][ 0] LogRHI: Using Forced RHI: D3D11
[2026.11.06-06.49.14:064][ 0] LogCsvProfiler: Display: Metadata set: rhifeaturelevel=“SM5”
[2026.11.06-06.49.22:617][ 0] Cmd: MAP LOAD FILE=“<PROJECT_ROOT>/Content/BoonieBears/Map/GouxiongLing.umap” TEMPLATE=0 SHOWPROGRESS=1 FEATURELEVEL=3
[2026.11.06-06.49.40:180][ 0] LogEditorServer: Finished looking for orphan Actors (0.001 secs)
[2026.11.06-06.49.40:809][ 0] MapCheck: Map check complete: 0 errors, 0 warnings.
[2026.11.06-06.49.40:842][ 0] LogUnrealEdMisc: Total Editor Startup Time, took 28.968
[2026.11.06-06.49.47:142][ 1] LogAssetRegistry: AssetRegistryGather time 0.2237s: AssetDataDiscovery 0.0255s, AssetDataGather 0.0591s, StoreResults 0.1391s. Wall time 28.4190s. NumCachedFiles 24292. NumUncachedFiles 1.
[2026.11.06-06.49.47:180][ 1] LogAssetRegistry: Display: Starting OnFilesLoaded.Broadcast
[2026.11.06-06.49.47:318][ 1] LogMetaSound: Display: Migrated Class Interface paged graph for MetaSound ‘/Game/Hyper/ResourcePack/Audio/MetaSounds/sfx_Weapon_BaseImpact_nl_metaPreset.sfx_Weapon_BaseImpact_nl_metaPreset’
[2026.11.06-06.49.48:695][ 1] LogAssetRegistry: Display: Asset registry cache written as 169.3 MiB to <PROJECT_ROOT>/Intermediate/CachedAssetRegistry/CachedAssetRegistry_*.ref

[HANG] Completed OnFilesLoaded.Broadcast is never logged. This confirms the issue is not specific to D3D12.

SUCCESS CONTROL - Bootstrap map, then delayed target-map load

[2026.11.06-06.57.48:133][ 0] Cmd: MAP LOAD FILE=“<PROJECT_ROOT>/Content/BoonieBears/Map/MainMenuWolrd.umap” TEMPLATE=0 SHOWPROGRESS=1 FEATURELEVEL=4
[2026.11.06-06.57.48:239][ 0] LogUnrealEdMisc: Total Editor Startup Time, took 9.950
[2026.11.06-06.57.50:088][ 0] LogPython: Waiting for Asset Registry before opening GouxiongLing (startup world: /Game/BoonieBears/Map/MainMenuWolrd).
[2026.11.06-06.57.50:395][ 1] LogAssetRegistry: AssetRegistryGather time 0.2343s: AssetDataDiscovery 0.0236s, AssetDataGather 0.0717s, StoreResults 0.1389s. Wall time 6.4210s. NumCachedFiles 24293. NumUncachedFiles 0.
[2026.11.06-06.57.50:414][ 1] LogAssetRegistry: Display: Starting OnFilesLoaded.Broadcast
[2026.11.06-06.57.50:451][ 1] LogAssetRegistry: Display: Completed OnFilesLoaded.Broadcast: 0.038s
[2026.11.06-06.57.55:516][ 17] LogPython: Asset discovery settled; opening GouxiongLing.
[2026.11.06-06.57.55:516][ 17] Cmd: MAP LOAD FILE=“<PROJECT_ROOT>/Content/BoonieBears/Map/GouxiongLing.umap” TEMPLATE=0 SHOWPROGRESS=1 FEATURELEVEL=4
[2026.11.06-06.58.13:615][ 17] LogEditorServer: Finished looking for orphan Actors (0.001 secs)
[2026.11.06-06.58.14:289][ 17] MapCheck: Map check complete: 0 errors, 0 warnings.

[SUCCESS] The editor remains responsive and PIE starts successfully.

Interpretation

The same map succeeds when loaded after Completed OnFilesLoaded.Broadcast but hangs when loaded before that broadcast finishes. This isolates the trigger to editor startup ordering around initial map loading, Asset Registry callbacks, and synchronous/async asset work. A debugger stack dump would still be required to prove the exact lock dependency.