This may greatly depend on the other duties the SENCO has.
For example if responsibility for planning interventions or carrying them out lies with the SENCO then a review of these plans or an observed group observation may be relevant.
In my experience the performance management can be based on a professional discussion and evidence/learning from the normal duties the SENCO carries out be used to support the identification of targets.
Objectives for the SENCO may relate to: the SENCO’s personal development or the SENCO’s professional development or the needs of pupils within the setting or the school’s improvement plan.
Ideas may for focal areas may come from official documents that explain the role and requirements of a SENCO, such as: the job advert used to hire to SENCO, the SEND code of practice (for a summary of the role of the SENCO in school see pp.108-109) or The learning outcomes for the National Award for SENCOs and the teaching standards (as some will still be relevant) etc.
Ultimately any targets and key learning will be specific to the individual and their setting.
Other places to find useful information include: area based documents (even if it isn’t your locality) such as: https://czone.eastsussex.gov.uk/media/7925/effective_senco_deployment_web-1.pdf
This may be out of date but can be useful if you are looking for how to breakdown the SENCO competencies for review: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/529271/Senco___Inclusion_Manager_Competencies_archived14june2016.pdf
I hope some of this is useful.