Why does placing a lid over a pot of water on a stove shorten the time it takes for the water to come to a boil, whereas, after the water is boiling, the use of a lid only slightly shortens the cooking time?

Atwo-stage air compressor operates at steady state, compression 10m^3/min of air from 100 kpa and 300k to 1200 kpa. an intercooler between the two stages cools the air to 300k at a constant pressure of 350 kpa. the compression processes are isentropic. a) calculate the power required to run the compressor, in kw b) compare the result to the power required for isentropic compression from the same inlet state to the same final pressure.