Are you running this test between RDS and MySQL workbench on your own machine?
The MySQL client (and MySQL workbench) are including the time it takes for RDS to return the value of "1" to your workstation (from my workstation on the other side of the world, the response from RDS US West takes 280ms).
If you try enabling profiling on the RDS service, you can see how long the query is really taking - it's completing on your RDS instance in fractions of a millisecond.
mysql> set profiling = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> select 1;
1 row in set (0.27 sec)
mysql> show profiles;
Query_ID Duration Query
1 0.00016200 select 1
mysql> show profile for query 1;
checking permissions 0.000008
Opening tables 0.000012
cleaning up 0.000003
12 rows in set (0.28 sec)
set profiling = 0;
Query OK, 0 rows affected (0.28 sec)