Gas water heater seems not to heat quickly

This year I have noticed that my water heater seems to run out of hot water more quickly than it used to. When I turn on the electric element it is drawing current and seems to be working. I checked on the temperature of the water coming out of the heater after the gas had turned off (after reheating) and it was about 140 deg. I let the hot water run until the temperature dropped to about 135 deg. At that point the gas came back on. I think this tell me the 12 volt gas thermostat is working ok. The flame seems to be adjusted right. Is it possible that there is a buildup of scale/calcium on the inside of the water heater keeping the water from heating as efficiently by the gas flame? I use a wand up into the drain hole of the heater once or twice a year and flush the white stuff out.

I may just be getting some quite cold water coming in from the cold water supply . It has been a cold wet spring here in Tennessee this spring. I just wanted to some suggestions..

Thanks

Ken