My phone is charging slowly

Check your micro usb port for dust and other types of grime first. If it’s clean then try using a different usb cable and/or power brick. If the cable and/or power brick charges a different phone fine, you have to check which version of MIUI you have. Basically anything after MIUI10 will have some sort of charging problem, especially the latest version of MIUI11. You either have to downgrade MIUI or install a custom rom to fix the problem.