if this is Cloud I would recommend you reach out to Atlassian Support as they are the only ones that have the necessary System Admin access to look into this. If Server then reach out to your system admin. There are many reasons that could cause performance issues (memory issues, VM sizing, runaway processes, network congestion, etc.).
You can check memory size from the system info page to check if you need to increase it.
if you would like to perform some troubleshooting to know the main reason you can follow this Best practices for performance troubleshooting tools
if you didn't get the reason, i recommend to communicate with Atlassian support
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