Theme Speed

Hi!
What plugins and cache do you use for the demo?
For the home page I saw a 1.78s load time and a very big page size of 3.4 MB.
Our shop I can’t reduce the load time under 2 secs despite the fact that the home page has only 2mb.
And how did you succed to have a great grade on the Combine External Javascript and CSS?
Thx!