Slowing D7 site..

Hi,

We have over the last couple of years built what is essentially a web based database application for our business that is accessed by our customers.. Launched in Sept 2011 and actively developed with new functionality being added since then.. We now have just under 16,000 "nodes" in the database..

The server is a Linode 1.5GB VM and runs nginx, php-fpm with APC and mysql..

Now I have to make it clear that its not a high traffic site with a peak of about 8-10 logged in users at a busy time.. Usually more like 3-4.. There maybe a few anonymous users visiting during the course of a day but probably not more than 1 at any point in time..

The system was always very "snappy" and page loads were fast but it seems that in the last month or so the performance has really dropped off significantly.. There has been no real configuration change other than updating D7 core and any module updates and the data hasn't grown any quicker than usual either..

The server doesn't appear to show any processor, memory or IO bottlenecks.. I suspected that the Linode host might be congested and Linode recently moved the VM to another server to see if that helped.. It didn't..

I have done some MySQL tuning but still didn't see any real improvement..

So I am not quite sure where else to look to work out what is causing the issue.. Or to figure out if it's caused by one of the core updates or a module updates..

For those of you with sites that have a large number of nodes are there any specific things that need to be done when passing say 15,000-20,000 nodes to help performance? I didn't think that 15,000 items what that much really but it maybe..

Any suggestions?

Thanks..