The best Slack apps for performance reviews are Effy AI and Teamble if you want reviews completed where people already work, and Lattice or 15Five if you want a full performance platform with Slack as the notification channel.
Effy AI syncs your Slack workspace, sends review reminders as DMs that open the form, and captures kudos with /create note; it starts at $3 per seat a month and is free up to 5 users. Teamble runs feedback inside Slack and Microsoft Teams and has a free plan. Polly is a strong Slack survey tool, but it does not do performance reviews.