diff --git a/MoneyMap/Pages/Upload.cshtml.cs b/MoneyMap/Pages/Upload.cshtml.cs
index 33231d0..1dc8858 100644
--- a/MoneyMap/Pages/Upload.cshtml.cs
+++ b/MoneyMap/Pages/Upload.cshtml.cs
@@ -255,6 +255,11 @@ namespace MoneyMap.Pages
             var previewItems = new List<TransactionPreview>();
             var addedInThisBatch = new HashSet<TransactionKey>();
 
+            // Load all existing transactions into memory for fast duplicate checking
+            var existingTransactions = await _db.Transactions
+                .Select(t => new TransactionKey(t.Date, t.Amount, t.Name, t.Memo, t.AccountId, t.CardId))
+                .ToHashSetAsync();
+
             using var reader = new StreamReader(csvStream);
             using var csv = new CsvReader(reader, new CsvConfiguration(CultureInfo.InvariantCulture)
             {
@@ -279,7 +284,8 @@ namespace MoneyMap.Pages
                 var transaction = MapToTransaction(row, paymentResolution);
                 var key = new TransactionKey(transaction);
 
-                bool isDuplicate = addedInThisBatch.Contains(key) || await IsDuplicate(transaction);
+                // Fast in-memory duplicate checking
+                bool isDuplicate = addedInThisBatch.Contains(key) || existingTransactions.Contains(key);
 
                 previewItems.Add(new TransactionPreview
                 {
@@ -415,7 +421,7 @@ namespace MoneyMap.Pages
             return PaymentResolutionResult.SuccessAccount(account.Id, account.Last4);
         }
 
-        private async Task<PaymentResolutionResult> ResolveAutomaticallyAsync(string? memo, ImportContext context)
+        private Task<PaymentResolutionResult> ResolveAutomaticallyAsync(string? memo, ImportContext context)
         {
             // Extract last4 from both memo and filename
             var last4FromFile = CardIdentifierExtractor.FromFileName(context.FileName);
@@ -425,26 +431,26 @@ namespace MoneyMap.Pages
             if (!string.IsNullOrWhiteSpace(last4FromMemo))
             {
                 var result = TryResolveByLast4(last4FromMemo, context);
-                if (result != null) return result;
+                if (result != null) return Task.FromResult(result);
             }
 
             // PRIORITY 2: Fall back to filename (for account-level CSVs or when memo has no card)
             if (!string.IsNullOrWhiteSpace(last4FromFile))
             {
                 var result = TryResolveByLast4(last4FromFile, context);
-                if (result != null) return result;
+                if (result != null) return Task.FromResult(result);
             }
 
             // Nothing found - error
             var searchedLast4 = last4FromMemo ?? last4FromFile;
             if (string.IsNullOrWhiteSpace(searchedLast4))
             {
-                return PaymentResolutionResult.Failure(
-                    "Couldn't determine card or account from memo or file name. Choose an account manually.");
+                return Task.FromResult(PaymentResolutionResult.Failure(
+                    "Couldn't determine card or account from memo or file name. Choose an account manually."));
             }
 
-            return PaymentResolutionResult.Failure(
-                $"Couldn't find account or card with last4 '{searchedLast4}'. Choose an account manually.");
+            return Task.FromResult(PaymentResolutionResult.Failure(
+                $"Couldn't find account or card with last4 '{searchedLast4}'. Choose an account manually."));
         }
 
         private PaymentResolutionResult? TryResolveByLast4(string last4, ImportContext context)