[Zope] Zope zserver-threads minimum is 5?

Hi all, My zope clients (Zope2-2.12.11) allways start with 5 theads although zserver-threads is 2 in my zope.conf file. I am using "ps -eLf" for theads counter. If for zserver-threads I use the values 1, 2 or 3 I get 5 theads. A higher value give me 2 threads more. I mean, for 4 I get 6, for 15, 17 threads, etc. It's correct? There are allways 5 threads minumun and 2 extra? It's a plone site. I am using zeo with separated catalog. I have 16 instances in a 8 cores machines and I think the GIL is making a poor performance. Thanks