my.cnf (5.5 and 5.1 is the same) :

back_log=200
max_connections=512
max_connect_errors=999999
key_buffer=512M
max_allowed_packet=8M
table_cache=512
sort_buffer=8M
read_buffer_size=8M
thread_cache=8
thread_concurrency=4
myisam_sort_buffer_size=128M
interactive_timeout=28800
wait_timeout=7200

mysql 5.5:

..mysql5.5/bin/mysqlslap -a --concurrency=10 --number-of-queries 5000 
      --iterations=5 -S /tmp/mysql_5.5.sock --engine=innodb
Benchmark
        Running for engine innodb
        Average number of seconds to run all queries: 15.156 seconds
        Minimum number of seconds to run all queries: 15.031 seconds
        Maximum number of seconds to run all queries: 15.296 seconds
        Number of clients running queries: 10
        Average number of queries per client: 500

mysql5.1:

..mysql5.5/bin/mysqlslap -a --concurrency=10 --number-of-queries 5000 
      --iterations=5 -S /tmp/mysql_5.1.sock --engine=innodb
Benchmark
        Running for engine innodb
        Average number of seconds to run all queries: 13.252 seconds
        Minimum number of seconds to run all queries: 13.019 seconds
        Maximum number of seconds to run all queries: 13.480 seconds
        Number of clients running queries: 10
        Average number of queries per client: 500

Why mysql 5.5 slower than 5.1 ?

BTW:I'm tried mysql5.5/bin/mysqlslap and mysql5.1/bin/mysqlslap,result is the same