set profiling=1, execute query, set profiling=0;

with the explain extended, and explain

without extended it take ~0.0014s  (in my query test, a union query with two queries with two tables and a single join)

with extended it take ~0.0019s (increase 0.0005s, 35% of overhead for explain)

maybe 0.5ms  isn't a bad thing on some queries
i will check more about the query rewrite of explain extended, but i think it's not a big problem on some workloads