Do invalid users and users that are marked disabled cause an impact on performance?
Not anymore! Previous versions (prior to 7.1 I think) users that were marked disabled were still processed in the post process authority (role) processing. If a user was no longer a valid user, say they had left the company and their user id was no longer valid, this could of impacted performance. Stratum now ignores any disabled user so that they do not impact performance. This allows you to disable but not delete users from Stratum.
If you do delete users Stratum will allow you reassign objects (like views) that they owned to a new user.
You can see if a user is disabled and if there network id is invalid in user maintenance in Stratum.