Let's deal with myths first:
1. More powerful charger: You can put your phone on a 500A charger, but it would still take between 500-1500ma, because the amperage is regulated by kernel, Sony's thermanager and is temperature dependent
2. Full charge in 1.5 hours: If your phone gets a full charge in 1.5 hours, your battery has a reduced capacity and is dying. Let's say the new battery has 18 cells and it should take about 3 hours to fully charge. If the battery is down to 4 cells, it would take 30 minutes to charge, but then it would discharge under load quickly
The main culprit is Sony thermanager. It is calibrated to severely limit charging current with heat and Z1 is known for overheating on a standard stock and CM kernels. I am aware of one kernel only - M5 which provides a multitude of adjustments that allow to keep the temperature down. You also need to adjust values in thermanager.
Take a look at my thermanager. Compare it with Stock/CM, but don't use with standard stock/cm roms, as it is modified for different CPU frequencies available in M5.