Hi,
Please check your customization like Workflow / Plugin, etc. which include on the Activities, Account and Contact entity. Also, try to deactivate / disable your Workflow / Plugins, etc. and check if you see any improvement in the performance.
Can you let me know whether you are using On-Prem or Online version of Dynamics CRM?
In case if you are using On-Prem, there could be a chance any background activities / jobs are running at the DB level, which causing this performance issue.
Hope this will guide you to the right direction.
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Thanks and Regards
Sandeep Sharma