SQL query to find amount of interactions per contact. More the number (1000+ of interactions per contact can degrade the performance.
SELECT TOP (100) ContactId, COUNT(ContactId) as Count
FROM [xdb_collection].[Interactions]
GROUP BY ContactId
ORDER BY Count DESC
So if you have got interactions more than 1000 per contact , you should purge the interactions which is mentioned in this blog.
https://www.giteshportfolio.com/blog/sitecore/november-2022/sitecore-delete-xdb-xconnect-collection-data-from-