The lag and A/C compressor thing *could* be related. If you have a bad pulley that's causing more friction than the obsene amount that our compressors already do, it could cause a bit more lag in power.
A/C takes a HUGE amount of power away from the motor. It's unbelievably noticable on Subaru's for some reason, so I would look into that.
It sounds to me like your car likely just needs a little TLC, replace spark plugs if you haven't already, and do your injector cleaning, make sure you replace all of the gaskets as you go along. MAF sensor COULD be gunked up as well.
As for your heater, you may have something gunking up your heater core inside of the dash. How does your temperature guage look when you're driving? If it's too low, you may have a stuck thermostat.