Check the temperature readings (Kernel Adiutor / EX Kernel Manager) and the CPU max. when charging - probably the kernel is throttling the SoC because of heat.
Probably you don't even realise it when it's in your palm,
that's why I suggest to check the actual readings
because the background process(es) are using the actual figures, not the feeling of your palm
and the Redmi Note 4 SD is (in)famously known to have a
bad suboptimal thermal engine config.
There's actually a very good Magisk Mod for the Mido in Magisk Manager Repo to change the thermal engine config (I opened an issue #2 because on MIUI it's probably not working as expected but I guess the dev. will change+update it soon (I did it for myself and looks promising).
Probably VDD_restriction just kicked in and limiting the CPU at 1.6GHz..
Not sure you have Magisk or not..
If not you can copy the thermal engine config file to the appropriate path from the module (
GitHub link here)
