My 1999 F150 Super cab (5.4) takes a while to heat up. The thermostat has been replaced and seems to work correctly. Once it does get warm, it blows good hot air (after about 15-20 minutes).

Any ideas to improve heater performance?

 

I was experiencing about the same problem on my 98 F-150 4.6L. My problem was that I would only get heat when the engine was turning about 1800 RPM's (Interstate). Then, once I came to a stop no matter how long I had been driving, the heater temp would drop to the outside air temperature. I asked a local mechanic what it could be and he said to check the coolant level. I found out I had a very low engine coolant level. Something, I never check when changing the oil. I added a gallon of coolant and now my truck heats up with 5 minutes at idle given a 50 degree day. After that, I took it to the dealer to have them purge the system since I noticed rust bubbling back at me when I went to pour in the additional coolant. I told the technician what had happen and he said they have seen leaking heater hose clamps next to the firewall. Sure enough, that was were I lost my coolant. They replaced the hose, the sticky thermostat, the clamp and the coolant. Good luck.