This does not sound normal at all. I have a similar set up to you in my development environment (in fact less memory and CPU but same OS/SQL) and do not see such performance problems. I never (rarely) add R/W SNMP strings when I use discovery and Add Node, so this is a new issue. What is using up CPU in your environment, or is it just slow without any visible resources constraints?
Please open a ticket and we can look at your issue first hand to see what might be causing a slow down and the strange behavior.