I'm working on reducing my performance quota and would like to know how to identify the transactions that are contributing to its usage.

On the Insights page, you can find the average transactions per minute (TPM), which is a good indication of your busiest transactions. Sort the TPM column to quickly identify high-traffic transactions.
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Warning: You can only update the columns of the Results table in Discover if your organization is on a Business plan.

You can also set up a query in Discover to take a more proactive approach to resolving your busiest transactions. When you're building the query, move to the transactions tab and then set the columns title, project, and count(), as shown below:

Once the changes are applied, sort the "Results" table by the "COUNT()" column to display your busiest transaction. From there, you can filter and/or sample transactions as needed. For more details on this process and on managing your performance quota, see the Manage Your Span Quota documentation.