Sounds a bit like the cylinder is slowly filling with sludge at the bottom and taking up the space previously used for water – when this happens the burner is trying to heat through the cylinder then the sludge to get to the water and in doing so it becomes less efficient and slower heating up.
Try draining and flushing the cylinder but turn the gas off to the burners first.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians