Hot water?

Hi

Live on the the ground floor and have a very stange situation.

When I switch the hot water in my kitchen it takes 6 seconds for the water to come out.

How ever if someone is taking a shower and then I open the hot water in the kitchen then it opens straight away.

The pump is a brand new 2 bar watermill and taps are made to cope with the preasure supplied.

Any ideas why this happens