Hi, Henry. I have done what you said. The plugins are necessary on my web site. Removed plugins and theme which I do not use. Am using Twitter Bootstrap already. As you can see, BuddyPress and s2members take most of time for loading. The others are less than 30%.
I deactivated BuddyPress and s2members, and my web site turned fast. That means these two plugins make my site slow. Hope the new version would provide better performance.