SNMP timeout in nagios

That the check can't contact the network service that it uses to monitor your server. I'd expect it's either WMI or NSClient. You haven't said that the host is down, so I'll assume that nagios can ping the host okay. If the service wasn't running, you'd get a connection refused message, but you're getting socket timeout, which means it's failed to establish a connection within a certain amount of time. Given that you can ping the machine and that you can't reach the socket, I'd say there's probably a firewall in the way.

I've always believed the administration advantage of SSH (I use push_check) outweighs any additional load. Modern CPUs are so fast that the cost of encrypting a handful of bytes is pretty minimal, so it comes down to running two processes (SSH and the check script) vs one (check script fired off by NRPE).

For check scripts written in an interpreted language, I would expect the overhead of firing up the interpreter (Perl, Python, Bash) to exceed the CPU cost of starting an SSH session. Given modern CPUs, your machines are more likely to be disk or memory limited rather than CPU limited.

Provided your Nagios machine is coping -- it has to set up 20 SSH connections every second -- I would err on the side of convenience.

Not really an answer to your question, more of an argument that life is too short to worry about it :)