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Power Custom Fields PRO™ runs an SQL query every time when a user tries to pull up a screen that contains one of the custom fields. The performance of the custom field cannot exceed the performance of the query when run directly from the database. So, if you have a query that takes 10 seconds to run when running from the database console it will take 10 seconds to run for the custom field. So for instance if you have 5 thousand users who are running a 10-second query every time they go to create a new issue or edit an issue this might have an impact on the performance of the database and increase the response time of Jira when it is running its own queries against the database. This would be true of any database plugin custom field, not just those developed by cPrimeCprime.
There are some design changes that could be used to mitigate the performance risks. For example, you could run the custom fields from a database cluster that is separate from the main Jira database. Also, using normalization on the tables and indexes targeted to the way the custom fields will be searching the tables will increase the performance of the custom fields.