Android

Assume from the various pd fun that could be occurring, it could be voltage related. Android phones often set their charge current by the input voltage. If the voltage sags, the phone assumes it is overloading the cable and backs off. If the Iphone and Android phone are sharing a 5V rail inside the charger then this could be an issue. If the combined load of both phones causes a voltage sag, the android phone will back off. However, if the Iphone doesn't also slow down at the same rate, you will get a bias. It might be that 90% of the power is going to the iPhone simply because the android phone is being polite, and being screwed by the deal.

The best way to test this is to install a charge monitoring app on the android phone. I personally use AccuBattery. It shows me both the voltage and current draw of my phone. The pulsing effect of a bad cable is very obvious with it. I suspect it would help you diagnose what is going on.

Does the charger display how quickly it is charging? I assume her iPhone's battery might just have a lower capacity as iPhone usually have smaller batteries than comparable Android devices.

Could it be as simple as the iPhone having a smaller battery?

Are they both using USB PD charging? Chinese brand phones often have weird proprietary charging protocols like OPPO VOOC for example.

Also if the ports are shared on the charger internally, plugging multiple devices in may limit what charging protocols and power levels are available.

It's not on purpose, but due to the complexity of USB and different charging protocols, and different charger designs.

Xiaomi makes Android phones, what would they get out of trying to make iPhones look better.

Most likely explanation is the voltage difference people have mentioned

there are a few factors to it for example your android might have a higher battery capacity

or her iPhone might support faster charging speeds

Just means it's a shit charger that can't handle multiple devices at once because that requires higher quality electronic components.

Are they both charging through a USB-C port? There are different levels of USB-PD specifications, and the charging speed depends on the maximum supported wattage of USB-PD between either the device, cable, or charger.

What's the model of your high end Android device, jf you don't mind us asking?

I've the exact same issue with my Baseus 100W power bank. It'll charge my S23 ultra super fast. Once I plug iPhone 14 pro, it'll charge it much faster than mine. Even if I unplug/plug again.

I think the issue with the power bank is because I've another 65W wall charger that'll charge both fast.

A potentially great way to destroy your GF's iPhone, iPhones start fast charging at I think 20W, the Chinese phones are like 45+ Some are even higher. Fast charging happens different ways, some hardware, some software, some a mix. Wrong charger with wrong phone can do serious damage. Not all internal circuitry will down volt what's coming in.