The heater is trying to warm up the water through a layer of sediment that has built up on the inside of the heater.
The bathroom farthest away from the water heater might take a bit longer because of the distance the water has to travel.
In water piping systems, usually at the end of a pipe run, an air chamber is installed to relieve water hammer. Sometimes a dishwasher with a quick acting solenoid will cause this, and in that case, a mechanical arrestor can be installed.
There could be an obstruction in the floor drain line. It could also be caused by the main sewer line.
In older homes, galvanized steel water lines corrode and narrow on the inside, causing reduced pressure or volume of water. In other homes, it can be a faulty pressure reducing valve or improper adjustment. Another possibility is an issue with a valve in the house controlling water flow.
On a lot of water heaters today, the problem is the gas valve/thermostat. If it is under warrant, the gas valve has no cost, and it will only require a labor charge to change it out.
It is good to keep up with the maintenance for a water heater. This would include draining it once a year, flushing out sediment, checking the anode rod, and cleaning the burner assembly.