Where is the bottleneck?

I have been noticing during busy times that it takes about 3-4 seconds to display a web page when it normally should 1-2 seconds. What I don't understand is that the background color (blue) and the header (including logo and menu links) are displayed immediately, while the table underneath the header table is blank until it finishes displaying a few seconds later.

During off-peak time the whole page loads together quickly. If I look at the source at the moment only the header is displayed, only part of the html is displayed; it's cutoff randomly at the bottom.

So my question is what could be causing this? Is the header part cached and the whole page hasn't processed yet? Or has the page been processed by the server and apache has a problem displaying the rest which includes about 30 thumbnail images and a bit of text etc. Thanks for any info on what may be causing this.

 

 

 

 

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