I would run the onboard diagnostics to try to focus the problem down. The battery sits on top of the keyboard and trackpad cables. Could you have nicked either one?
The slowness is expected when one of the SMC sensors has failed to communicate to the SMC logic. Your system is in CPU safe mode, the CPUs clocking is lowered and the fans will ramp up.
What happens when you use an external keyboard and mouse? If it’s stable then that aims us back to the cables and the connections to the logic board.
To get to diagnostics, restart and press the ***D*** key.