Most Mac OS aren't designed with low memory in design. If your device is hot then it is trying to do stuff awhile trying to deal with your low memory situation.
What to do?
Backup all your info.
Delete anything that you don't need to use. Make space so the device isn't trying to thrash and waste energy finding scattered memory fragments.
Your phone will need to be reset and restored with a fresh copy of iOS then replace your data and then see how it's behavior is doing.
Following.