Pumped shower take 5 mins to come hot

So I have a pump assisted shower fed from immersion tank, when I turn on the valve there is a 5 min wait till the warm water comes through even though the mixing valve is turned to hot the pump sounds like it's working normally. Is it possible there could be some kind of airlock , it just makes no sense! The shower and the pump are only a couple of meters away from each other. It was not always like this it used to come through very quickly! Any ideas ?? Getting major grief from family members

Is it just these cold days, the pipes are icy cold and it may be taking few minutes to heat them up.

Internal pipework is unlikely to be the issue unless you live in an extremely cold house with little heat, check a hot tap in the basin, if the immersion is on for a while and has had no draw from the tank then it should be hot, if not your immersion is the issue, you can adjust the immersion at the top of the tank to get your water hotter, if the pump is drawing water from the tank then the shower is ok.

Thanks for your advise pale rider and money box, so I took the valve apart and suspected the thermostatic control , I put them in an ultrasound to clean any debris and cleaned the inside housing of the valve, poped a bit of Vaseline on the o rings , reassembled and making sure the mixer at the back of the valve was riding properly on its spring load tightened it up and away it went. I suppose the moral of the story is that the thermostatic valve is sensitive enough and probably needs a service every few years, it took a while to do but better than a call out and ,or a new thermostatic at 120 euro a pop.