Loading speed problem

My website has a loading problem. So I need to get rid of some plugins and compress the images. But before I do that I would like to know which plugin causes the most load. For instance, if there is one plugin causing the all slowing down, then I would like to disable just that plugin, and I also do not have to compress my images as well. So do you know any method which detects the most troubled plugin causing the most slowliness on my site? Thanks.