If sealed pressure chamber contains only one condensable vapour, heat trans fer is remarkably fast. The vapour pressure of the vapour in the chamber is set by the coolest point, and the working fluid condenses there and wicks away to the hotter points.
If the there's a thick layer of condensed liquid at the coolest point, heat transfer through that would probably be the limiting factor, but the vapou r is is usually condensing into a woven copper cloth wicking layer.
The vapour itself moves very fast - it used to whistle inside my vacuum lin e when I encouraged a vacuum distillation process with a hot air gun.
A non-condensable gas leaking into the heat-pipe environment can slow the v apour flow a lot. At Iasys we used to test our heat pipes for rapid heat tr ansfer at close to room temperature to make sure that the manufacturer had got all the air out before putting in the water - initially we were sending back quite a few, but they got better.
Folk wisdom at the time was that heat pipes didn't work well at low heat di fferentials close to room temperature, but that was only true of badly made heat pipes.