The theory is that one should go much easier on the flats, using an effort that's below VT1 (google) so that one will have the energy to go harder on the hills, hard enough that one is breathing heavily going over the tops. Of course that depends on the height of the rollers. IME it takes at least 100' of steady rise to get a good response in terms of cardiac effort. If this seems interesting, you can try to reduce your time on a common route by going slower on the flat and faster on the climbs. That usually works, just don't turn it into a Thing. There's a balance there that's interesting to find. It can be something with which to amuse oneself.