> Our plan was to plug the new beast as a slave of our current master to watch how it behaved.
That's a good first step.
> If we install Mysql server 5.5 with default cnf
No. Tune it.
http://mysql.rjweb.org/doc.php/memory
> How on earth slave state can increase performance 10x ?
Caching. (mainly key_buffer_size)
Jørgen, did you create a new login? Or did something get mangled with your name?
> restarting Mysql lead to bad perf. again.
Cold cache. 10x slowdown is typical.
> Strange isn't it ?
No. "Count the disk hits". (Once you understand how MySQL's caching works.)
> Maybe becoming slave influenced its execution plan ?
Many things can influence the EXPLAIN; sometimes even the phase of the moon. (OK, not literally.)