Added comment.

data.c

@@ -566,7 +566,7 @@ int _object_user_cache(snac *snac, const char *id, const char *cachedir, int del
     int ret;
 
     if (del) {
-        index_del(idx, id);
+        index_del(idx, id); // expensive; let it fail on processing?
         ret = unlink(cfn);
     }
     else {