DOC: add hint for benchmark tests

doc/devel/Benchmarking.md

@@ -59,7 +59,10 @@ enabled on start up.
 This script expects the MariaDB server to be accessible on 127.0.0.1 with the default caosdb user
 and password (caosdb;random1234).
 
-
+You might consider to increase `performance_schema_events_transactions_history_long_size`.
+```
+performance_schema_events_transactions_history_long_size=1000000
+```
 The performance schema must be enabled (see below).
 
 ### MariaDB General Query Log ###