The machine is laggy and the SMC reset is not working
Have you tried to boot into single-user mode ( boot while holding down the command and the S buttons untill you see a black screen ) and then run " fsck -fy " command? 

This will run some diagnostics and make any changes needed. When it is finished it will say  drive appears to be ok OR changes were made. 

You then type " reboot " ( without quotation marks ) and it will reboot.

Troubleshoot Your Mac With These Hidden Startup Options

Use safe mode to isolate issues with your Mac