My Journey is like that... When I first got it 2 years ago, it was instant now it takes a good 5-10 minutes to start feeling the cold air. My dad has his mechanics and A/C license and put the gauges on it and it's pulling the right pressures. We tried adding and removing some refrigerant but no change.
If I turn on the Rear A/C with the front touchscreen it seems to cool faster, I think because it increases the duty cycle of the compressor and maybe opens another valve to feed the rear. I don't know...
Never had this problem with my Santa Fe in 300K Miles (535,000 km's) before the engine died.