Bouncing Zabaglione Bros. said:
One of the "standard" tweaks is to change how long Firefox waits on a page elements before trying to render it. Although this can speed up how a page appears (starts rendering instantly without waiting to get the rest of the page/layout), other pages with a lot of elements may appear to be slower because FF will have to keep redrawing the page as more elements come down the line. It could be especially bad on dial-up or with slow websites.
As I said above, the default settings tend to play it safe, trying to give a balanced browsing experience. There's nothing to stop you tweaking everything to extreme settings, and this program seems to do it all for you without bothering you with the details. It's just worth bearing in mind that the reasons why these arn't the default settings is because there is often a downside (depending on your configuration) to choosing these tweaked settings, that the average user might not understand.