[adgod]

Did you do the ROM flashing process by yourself? Even if, what phones did you do on? Are they Android version 5.0.0+? Because older ones doesn't have bootloader security and flashing from recovery terminal from sdcard is easier. If you don't know about it, you can get a in depth guide about it from here:-

https://www.xda-developers.com/how-to-in...m-android/

I've followed this process myself and can verify this information to be correct. If you're still unsure you can search your phone model up followed by "custom ROM flashing" and a number of guides will show up.
Secondly I really don't understand what you mean my "features". You mean customisable? Every brand modifies the base Android source code to create a modified version of the OS to properly communicate with their custom hardware. Custom ROMs may give a lot of more flexibility to the UI but it's a lot riskier attempt as there are a high chances of getting your phone hard bricked. Flashing a recovery at the first will be my opinion if you're going for it. I'll recommend TWRP:-
TWRP.me
Every phone with any ROM will first run smoothly and get laggier after months because of data cache being stored. It has nothing to do with the topic. Overheating is not even remotely related to the ROM or "custom kernel".