Question about fast chargers for iPhone XS Max
Hi everyone, from what I understood, an 18w charger and above is recommended for fast charging. I’ve been using my 12w apple charger to charge my XS Max, but recently purchased this [18w charger](https://www.amazon.com/dp/B0728HB18G/ref=cm_sw_r_cp_api_i_2v2HCbS7SNPS9). This charger charges much slower than my 12w apple charger even though it’s stated as 18w. Can someone explain to me why?
I also purchased this [39w Aukey charger](https://www.amazon.com/dp/B01BBZJ31Y/ref=cm_sw_r_cp_api_i_3x2HCbE335Z2Z) and it’s super quick, but very hefty to carry around in a small backpack, and doesn’t fit in my school’s desk outlets.
I’m looking for something portable, yet faster than Apple’s 12w adapter.