Google Webmaster Tools give you quite a bit of feedback on your websites. Although you didn't ask for a link, this is one that might help:
This tool shows you the download speed of any page of your website. It also analyzes the page, and this part gets pretty technical. But even if you don't understand what they are talking about in the analysis (I usually don't.) it is helpful to know how your webpage is performing, and what you might do to speed things up.
You have already explored some plugins, so I won't repeat what others have mentioned.
One of the main things is images. Before you upload images to a website, take some time to resize them. You don't want to make them too small, but definitely not too large.
Some WP themes are slower than others. Browsers read html codes. If there is too much html code before any content on your site, it can slow it down. Excess html code can come from the theme you are working with, as well as from other programming.
Too many plugins can also make the site a little heavy. I remove plugins and themes I am not using. It seems to help.