[TYPO3-english] speed up page load - what is ...cached objects/Substitute header section?

[Stefan Kaufmann]

Dear list,

I have occasionally Gateway Timeout errors on my site and the hoster 
recommended me to check with the admin panel to see what slows down the 
website. Although not clear yet what causes the Gateway Timeout exactly, 
I discovred that the followingg slows down my entry page:

...cached objects/Substitute header section

What does it mean? Why would this slow it down so much?
What can be done about it?

Other pages load within 1 second, but the main entry page needs often 
4-5 seconds.

Many thanks for your advice.
- Stefan Kaufmann