High TTFB on cart / checkout pages

So recently we were succesful on reducing initial server response time on most of pages on our store with caching and some optimisation (we are still on the way with image optimisation), however we were unsucessfull with dealing with slow cart and checkout pages, as they can’t be cached. What techniques can possibly be used to aid us with this conundrum?

The page I need help with: [log in to see the link]