My xiaomi portable fast charger clearly charges my girlfriend’s iPhone faster than my high end android device. Doesn’t matter which port, which cable or which android phone. It starts by charging fast but as soon as I link her phone the fast charging speeds go straight to hers and mine just chargers very very slowly.
I’ve got the impression that this may be on purpose or at least has some clear explanation.
Any thoughts?
Yup. Some chargers support 5V@3A (15W), others 15V@1-4A (15,30,45,65W), 20V@3.35A (65W), some various amps at 9V,11V etc
Some chargers support all of the above voltages at various amps. Others only a subset.
If the OP has a Pixel, those seem to use 9V up to about 2.5A. iPhone similarly does “fast charging” at 9V (apparently up to 3A or so). The Android should charge test, so the limiting factor might actually be the cable. Some older cables don’t support or allow the charger to negotiate higher voltages so the Android cable may be holding things back.