Slow Keys at log-in screen means I can't access laptop!
If you did a clean install, that would suggest a hardware issue. It's possible you've got a corrupt install media, too. This can happen if you've created a USB flash drive installer and the USB drive you have doesn't like the block size you've specified (or the default) when using the dd command.

Don't forget to reset the PRAM, as well as the SMC. Apple's instructions:


Shut down your Mac.
Find Command (⌘), Option, P, and R on your keyboard.
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you
hear the startup sound.
Hold these keys until the computer restarts and you hear the startup
sound for a second time.
Release the keys.