It's possible hard disk errors might be causing some of the symptoms you describe - assuming the hardware is up to specification.
Checking SMART stats might show something.
There's not much you can do about delays during install but the command
systemd-analyze blame
Will break down what happened during a boot by the time it took. Running that command after a long boot and comparing the output with similar from a normal boot might be a lead in to what is causing the boot delays.