Common Review Date and System Performance

I would be interested in speaking with other customers who have more than 20,000 employees and have ePerformance implemented.  When we first went live with ePerformance, we had only a small number number of our employee population on a common review date and the vast majority on anniversary date.  Since our 2014 implementation, we've steadily increased the number of employees on the common review date and rapidly the last two years.   As a result, we've seen performance degradation during the common review period with the spinning wheel. 

1- How have others load tested to ensure they can handle the increased access/usage?