As a manager, you have an employee who is not doing very well at work. Performance-wise he/she is an outlier but he/she has a very good adherence.?

I would talk to him/ her through a one-to-one meeting and have an open discussion to know the actual reason behind this issue, in order to try solving it within my possible domains.

Then moniter closely the effect of that meeting to ensure everything is going towards the correct path.

I would like to take a turn on this Question.

If a person is not good at her performance,try to find if its a will issue or skill issue.

If its will issue take action after giving verbal warning and written warning with her manager's involvement.

If its skill issue,give a TNI (Training need identification).

When i talk about TNI,As a manager we need to identify what is impacting her work.

If there is personal issue which she would like to share,help her to understand balance work life.

Check for ADHD. This is the main medical reason in this case. 

Multiple other factors can also be contributing, including personal health, personal issues at home most important lack of interest.