[DOCS] Adds performance considerations section to transforms overview (#53791)

Co-Authored-By: Lisa Cawley <lcawley@elastic.co>

docs/reference/transform/overview.asciidoc

@@ -62,3 +62,19 @@ image::images/pivot-preview.jpg["Example of a {transform} pivot in {kib}"]
 IMPORTANT: The {transform} leaves your source index intact. It
 creates a new index that is dedicated to the transformed data.
 
+
+[[transform-performance]]
+==== Performance considerations
+
+{transforms-cap} perform search aggregations on the source 
+indices then index the results into the destination index. Therefore, a 
+{transform} never takes less time than the cumulated duration of the 
+aggregation that it performs and the indexing process.
+
+For better performance, make sure that your search aggregations and queries are 
+optimized, so they don't process unnecessary data.
+
+NOTE: When you use <<search-aggregations-bucket-datehistogram-aggregation>>, the 
+queries are not considered optimal as they run through a significant amount of 
+data. For this reason, {transforms} performing date histogram aggregations take 
+longer to run.