Waterproof membranes (Gore-Tex, eVent et al) breathe better when the membrane is kept warm and there is a large temperature and humidity difference between the inside and outside (as shown in these lab results). In hot and humid conditions external and internal conditions are similar, reducing breathability.
A thin hardshell offers little insulation and by wearing it near the skin you can keep cool, reducing the amount of sweat and humidity you produce. This also keeps the membrane warm, making condensation less likely. However, when the Durable Water Repellency wears off or condensation occurs in the outer fabric, the membrane becomes chilled and breathability reduces.
In my experience it is only a matter of time before you get wet... and progressively cooler.
(Adding insulation under a waterproof when you stop is a problem when it's raining - try an OverShell or Shelter.)
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