If you’re set on milking as much out of your time in Cambodia, Laos and Thailand as possible, then there’s a number of ways to save time.
Travel at night
In Thailand, when you’re travelling out from Bangkok you can reach anywhere south of Chumphon, north of Phitsanulok, anywhere near the Mekong River and Trat (for Ko Chang) by travelling by night-bus or night-train. In particular there is a handy bus from Bangkok to Trat which leaves just before midnight getting there around 6am the next day, leaving you time for a snack in the market before getting the ferry to Ko Chang. Closer destinations are too near to Bangkok to allow for a full nigt of sleep.
In Laos, there are long distance night buses — particularly from Vientiane heading south. We don’t recommend the night buses in the north because the roads are poor, street-lighting non-existent and the buses tend to have one driver only.
In Cambodia there is no overnight transport. In Vietnam both trains and buses are popular overnight options — particularly Hanoi to Sapa and the long trunk routes north and south along the long Vietnamese coast. Train is generally the better, more comfortable option.
Easily the biggest time-saver and with the growth of Low Cost Carriers (Air Asia and Nok Air in particular) it is both affordable and very fast to fly between destinations. The further in advance you book, the cheaper the tickets become — often cheaper than train or bus