As part of our continuous performance improvements, we've improved several areas from reports/gadgets to functions.The most intensive task related with calculation of coverage is now how the status coverage is pre-calculated, which should provide quite noticeable performance improvements.
In sum, we've improved performance on:
Test Lists gadget;
Overall Requirement Coverage gadget & report;
Historical Requirement Coverage gadget & report;
requirements() and requirementsWithStatusByTestPlan() JQL functions
noticeable performance improvements inherently affecting most operations that trigger this calculation.
Improving performance is an ongoing effort, so expect additional enhancements to come ahead.
We also made an extensive document with Usage tips to improve performance, containing setup and usage best practices; please do take a look at it.