When you connect to this data, Tableau will interpret it as a string and it will be available in the Dimensions pane. If you don't need the 'idle' values at all, simply right click the field and select 'Change Data Type > Decimal'. Tableau will automatically return any non numeric values into a null so your average will work fine.
If you want to keep the performance field as a string so you can still analyze the 'idle' instances, create a calculated field to create a 'numeric' version of this field. I.e. PerformanceNumeric = float([Performance])
Similar to above, this will drop out non-numeric instances in the field and replace them with null so you can run an average on the new PerformanceNumeric metric by adding the performancenumeric measure to the report and changing the aggregation to AVG.
Thank you Tom! Changing the data type to decimal did change the numeric values to null.