Performance test server side

Hello,
My wordpress blog is hosted on a webserver in my NAS (Synology).
My problem is that the loading time is really slow. For example, when I test my site with “tools.pingdom.com”, the first access is really slow. I get the timing just for the 1st request :
– DNS : 2 ms
– Connect : 36 ms
– Send : 0 ms
– Wait : 16 300 ms (16 seconds)
– Receive : 116 ms

After all the element load quickly (load time 19.46 sec). I’ve tried some cache system (WP Super Cache or W3 Total Cache) but it didn’t help. I tried to disable all the addons still the same.

So I’m looking for a tool or a plugins that can tell me what happen during the 16 seconds first wait. Maybe a plugins that analyse thing on server side. Can you help me ?
Perhaps it’s the gzip function ? Synology server are not really fast but it strange that it can take so long just to wait.

Regards.