Slow Response Times – how to check what is causing it

My wordpress site has been getting really sluggish response times over the past few weeks, and is timing out and giving some people 404 errors.

It used have response times approx 600ms and is now around 2700ms, peaking at over 4000ms.

I have contacted the hosting company and they have found no issues their end, they setup a new test site as an addon domain and response times with this new site are good.

So it seems like there is an issue my end and with my site.

My question is how do I find out what is causing this?

Is there a way to see what scripts etc are causing the load?


WOrdpress: Latest Version

Theme: Thesis 1.8.4 with Marketers Delight Skin

Approx 9 plugins

I did add a security monitoring plugin for a while but removed it as this seemed to slow things down.

I appreciate your help with this one.

Regards, Tom