slow log in macbook pro

Even though your Mac obviously turns on and you can log in, begin with If your Mac doesn't turn on. That gets some preliminary steps out of the way.

Read and follow each of those steps, including all the links within it, all the way through and including Step 4, which directs you to:

If your Mac starts up to a blank screen

Read and follow each of it steps also, all the way through and including the end, which takes you to:

"If you still need help, please contact Apple Support."

Run the Apple Diagnostics.

Try booting into Safe Mode.

Try booting into Internet Recovery Mode using Command + Option + R and run Disk Utility First Aid on the drive.

You may have a failing hard drive. You may also have a defective hard drive SATA cable which has an extremely high rate of failure with this particular Mac.