Slow is more stressfull

Actually the slow performance, even if only prevalent in parts of the site, becomes a defining attribute to the whole site. Concentrating web performance efforts only on frequently touched landing pages, prominent entry points or other singular pages, makes a non-sustaining impression.
To tackle the issues with a slow performing web experience, don't forget to analyse the performance of your media or image server. A single image can block a whole page load process. I recommend some of the tips taken from the toolkit Intermedia Fotoficient.