diff --git a/modules/wafrn.nix b/modules/wafrn.nix
index e11ec40..bdfe939 100644
--- a/modules/wafrn.nix
+++ b/modules/wafrn.nix
@@ -456,6 +456,7 @@ in
       serviceConfig = {
         Type = "oneshot";
         RemainAfterExit = true;
+        TimeoutStartSec = "30min";
       };
       script = ''
         set -euo pipefail
@@ -478,10 +479,25 @@ in
             exit 1
           fi
 
-          rm -rf "${preparedSourcePath}"
-          mkdir -p "${preparedSourcePath}"
-          cp -a "$selected_source/." "${preparedSourcePath}/"
-          chmod -R u+w "${preparedSourcePath}"
+          marker_file="${preparedSourcePath}/.wafrn-source-origin"
+          need_refresh=1
+          if [ -f "$marker_file" ]; then
+            if [ "$(cat "$marker_file")" = "$selected_source" ]; then
+              need_refresh=0
+            fi
+          fi
+
+          if [ "$need_refresh" -eq 1 ]; then
+            echo "wafrn-nix: preparing source tree from $selected_source" >&2
+            rm -rf "${preparedSourcePath}"
+            mkdir -p "${preparedSourcePath}"
+            cp -a "$selected_source/." "${preparedSourcePath}/"
+            chmod -R u+w "${preparedSourcePath}"
+            printf '%s' "$selected_source" > "$marker_file"
+          else
+            echo "wafrn-nix: reusing prepared source tree" >&2
+            chmod -R u+w "${preparedSourcePath}"
+          fi
 
           if [ ! -e "${preparedSourcePath}/.git" ]; then
             mkdir -p "${preparedSourcePath}/.git"