Hi @Bhupender Kumar ,
From what I read, we usually check the status of both "RPC Operations/sec" and "RPC Requests" when troubleshooting slow RPC request processing issues. So you could check the RPC Requests performance counter as well and see how it goes.
- If RPC Requests is steadily increasing while RPC Operations/sec remains fairly stable, this indicates that the server cannot handle the existing workload and it's suggested to verify hardware components or decrease the number of users on the server. (This doesn't seem to be applicable to your situation as your hardware configuration seems good enough and you have tried to leave a single mailbox on the server.)
- If RPC Requests is steadily increasing but RPC Operations/sec steadily decreases, this indicates that the Exchange server is the source of the bottleneck. This kind of situation is generally caused by either a serious physical resource shortage (memory or disk) or a processing issue within the Information Store or an integrated component (antivirus, journaling, and so on).
Reference: Troubleshooting Slow RPC Request Processing Issues.
Furthermore, for current situation, if the users who report the performance issues are running Outlook in Online mode, it's recommended to switching to Cached mode instead which could potentially help improve the performance.
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