I just solved a similar issue with a clients site. I generally did a few things to get the sites dashboard to respond quickly.
1. Make sure your theme is not looking for updates. Some themes have major issues performing a handshake with theme developers to check updates. So you can find the line in your functions.php file and comment that crap strait out, or if your lucky just turn it off in your themes setting if they give you the option.
2. Turn off all your plugins and turn them all back on one at a time. This may be a source of problems and could solve your problems so as painful as it is when the back-end is slow do it. If your familiar with PHPmyadmin then google how to disable your plugin there its way faster.
3. Make sure you have allocated enough PHP memory on your hosting account. If you have no idea about this then just contact your hosting support and ask them to "increase memory allocated to PHP" just tell them you are having some problems with plugins in your WP site and they will take it from there.
4. (shout out to Songdogtech ^) Cut down on unnecessary plugins. When you remove a plugin me sure to drop its tables in PHPmyadmin. Don't touch PHPmyadmin unless you know how to back up and restore at least...
There are a few more steps... I think i'll blog about it but the things I've listed here should help some people. Good luck and i'll come back and add a link to my full blog post when i get a chance.