I know you posted this a while ago but I didn't have this issue until I upgraded to the 17G2208 build of High Sierra. I tracked it down to the Biometrics support in OSX. It's looking for the Biometrics hardware (Fingerprint scanner and what not) that doesn't exist and keeps trying to do so for a few times before it gives up, which is why you get the system lag/beach ball when going to enter in a password.
My guess is everyone running this build will have this laggy password entering issue, so here's the fix that I figured out for it.
Since we don't have any kind of biometrics hardware support on this laptop, the simple fix is to navigate to System-Library-Frameworks and delete the following framework extensions:
BiometricKit.framework
BiometricKitUI.framework
BiometricSupport.framework
I made a backup of them, just incase, but I can't ever see a point where they will be needed. But after you delete them, reboot and you won't have the lagging issue when entering in your password anymore.