Maybe you are confusing yourself:

“The problem now is that Super Server is not usi ng the full (SMP) processing power of the hardware, than slowing things down”

“So, we need to get back to Firebird 2.1 Classic but could not figure out how to do that (tried some options without success).”

So, you know you need install FB Classic, but you said that you is using FB Super Server.

So, install FB 2.1 Classic for SMP support.