So slow you have to wait for the mouse to react (Mid 2012 MacBook Pro)

[Chadest]

Yes, it’s an old Mac. But what’s going on here? It’s been cleaned up, fresh install of El Capitan, then the battery died for the last time. It still runs off the power cord though. It’s just so slow, uselessly slow! You have to wait to register a button click, a key press, or even for the mouse to react.

I was advised that it should run fine via the power cord even with no battery installed, and that the problem was probably the hard drive cable. I replaced it and it DID NOT fix the problem.

Where do I start troubleshooting now?

 

[Audit13]

Without a battery, the OS will throttle the CPU.

 

[chscag]

Member @Audit13 is correct. Your MacBook Pro needs a working battery in order for it to operate normally. Replace the battery with a new one.