It sounds like the system is in CPU Safe mode. The CPU's clocking is reduced and the fan is pumped up both to protect the system from over heating.
Restart the system and enter into Diagnostics pressing the ***D*** key. Let us know what the errors are. This could be a simple issue where one of the ribbon cables didn't get seated properly or the batteries BMC logic is messed up.
Reference: [https://support.apple.com/en-us/102603|Mac startup key combinations]