Laos is many things but it's certainly NOT flat. Every day is either up, down, or up and down. Makes for slow cycling. My longest day has only been 120km and average about 100km but it's the elevation gain that slows things down – that and being sick all the time. Biggest day had 2000m (6000 feet) of vertical gain – that's over a mile straight up.
Today was about the flattest so far and even today probably had 3000 feet of climbing. This photo is overlooking the large reservoir immediately to the north of Vietienne.