diff --git a/README.md b/README.md
index 24dd3e8..7139b87 100644
--- a/README.md
+++ b/README.md
@@ -142,6 +142,9 @@ Count in the extra file is optional, since there isn't always a post count for c
 The extra files can also be used to just add new / custom tags not included in the main set, provided `onlyAliasExtraFile` is false.
 If an extra tag doesn't match any existing tag, it will be added to the list as a new tag instead. For this, it will need to include the post count and alias columns even if they don't contain anything, so it could be in the form of `tag,type,,`.
 
+##### WARNING
+Do not use e621.csv or danbooru.csv as an extra file. Alias comparison has exponential runtime, so for the combination of danbooru+e621, it will need to do 10,000,000,000 (yes, ten billion) lookups and usually take multiple minutes to load.
+
 ## CSV tag data
 The script expects a CSV file with tags saved in the following way:
 ```csv