Report performance bottleneck: alterCustomDataDisplay()

We are trying to export to CSV from the Mailing Details report, but encountering constant timeouts.

The biggest time-wasters are actually in preparing the data for display - particularly the alterCustomDataDisplay() method in CRM_Report_Form.

This calls CRM_Core_BAO_CustomField::displayValue() for each custom field value. So if you have a report with 100,000 rows, and 5 custom fields, that's 500,000 calls.

Depending on the custom field, the 'display value' is actually no different from the normal value, so in a couple of reports we've just been able to comment out the function without any detrimental effect.

This needs a bit more investigation, but there must be a way to streamline the alterCustomDataDisplay() method - either by joining on to the right value in the first place, or by caching the data...