diff --git a/modules/routes/routeAdminDatabaseHealth.py b/modules/routes/routeAdminDatabaseHealth.py
index a01c0648..b035eeda 100644
--- a/modules/routes/routeAdminDatabaseHealth.py
+++ b/modules/routes/routeAdminDatabaseHealth.py
@@ -479,6 +479,47 @@ def getMigrationExportDownload(
     )
 
 
+def _processUploadedFile(filePath: str, tmpDir: str, token: str) -> dict:
+    """Parse JSON, validate, remap, split into per-DB files.
+
+    Runs in a thread pool to avoid blocking the asyncio event loop
+    during the CPU-heavy json.load() of large (500+ MB) files.
+    """
+    import gc
+    import os
+
+    with open(filePath, "r", encoding="utf-8") as f:
+        payload = json.load(f)
+
+    try:
+        os.remove(filePath)
+    except OSError:
+        pass
+
+    result = _prepareImport(payload)
+
+    if not result.get("valid"):
+        del payload
+        gc.collect()
+        return {"result": result, "dbFiles": {}}
+
+    protectedIds = result.get("protectedIds", [])
+
+    dbFiles = {}
+    databases = payload.get("databases", {})
+    for dbName, dbData in databases.items():
+        dbPath = os.path.join(tmpDir, f"poweron_import_{token}_{dbName}.json")
+        with open(dbPath, "w", encoding="utf-8") as dbF:
+            json.dump(dbData, dbF, ensure_ascii=False, default=str)
+        dbFiles[dbName] = dbPath
+
+    del payload
+    del databases
+    gc.collect()
+
+    return {"result": result, "dbFiles": dbFiles, "protectedIds": protectedIds}
+
+
 @router.post("/migration/upload-import")
 @limiter.limit("5/minute")
 async def postMigrationUploadImport(
@@ -489,7 +530,7 @@ async def postMigrationUploadImport(
     """Upload a backup file to disk (chunked), validate, remap IDs,
     split into per-DB temp files so the full payload doesn't stay in RAM.
     """
-    import gc
+    import asyncio
     import os
     import tempfile
     import uuid
@@ -520,47 +561,31 @@ async def postMigrationUploadImport(
                 totalBytes, totalBytes / 1024 / 1024)
 
     try:
-        with open(filePath, "r", encoding="utf-8") as f:
-            payload = json.load(f)
+        processed = await asyncio.to_thread(_processUploadedFile, filePath, tmpDir, token)
     except (json.JSONDecodeError, UnicodeDecodeError) as e:
-        os.remove(filePath)
+        if os.path.exists(filePath):
+            os.remove(filePath)
         raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail=f"Invalid JSON file: {e}") from e
+    except Exception as e:
+        if os.path.exists(filePath):
+            os.remove(filePath)
+        raise HTTPException(status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail=f"Processing failed: {e}") from e
 
-    try:
-        os.remove(filePath)
-    except OSError:
-        pass
-
-    result = _prepareImport(payload)
+    result = processed["result"]
+    dbFiles = processed.get("dbFiles", {})
 
     if not result.get("valid"):
-        del payload
-        gc.collect()
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
             detail={"message": "Payload validation failed", "warnings": result.get("warnings", [])},
         )
 
-    protectedIds = result.get("protectedIds", [])
-
-    dbFiles = {}
-    databases = payload.get("databases", {})
-    for dbName, dbData in databases.items():
-        dbPath = os.path.join(tmpDir, f"poweron_import_{token}_{dbName}.json")
-        with open(dbPath, "w", encoding="utf-8") as dbF:
-            json.dump(dbData, dbF, ensure_ascii=False, default=str)
-        dbFiles[dbName] = dbPath
-
-    del payload
-    del databases
-    gc.collect()
-
     logger.info("SysAdmin migration upload-import: split into %d per-DB files, payload freed",
                 len(dbFiles))
 
     _pendingImports[token] = {
         "dbFiles": dbFiles,
-        "protectedIds": protectedIds,
+        "protectedIds": processed.get("protectedIds", []),
     }
 
     return {