Hi David,
Thanks for your interests in Syncfusion products.
We tried to host our FontListBox with a VirtualizingStackPanel with IsVirtualizing set to True and tried to calculate the time taken for loading the fonts, initially it took some time actually but once after it is initialized for the first time there is no issue with the performance of the FontlistBox. Kindly download the sample from the below location.
httP://files.Syncfusion.com/Support/Tools.WPF/7.2.0.37/Forum-84070/main.htm
Try reproducing the issue. Still if you have any issues kindly update us.
Being an evaluator We appreciate the opportunity to assist you. We might like to know that if you have an existing personal Direct-Trac account available using your own email address, and we highly recommend using your personal account. You'll get a faster response along with an automatic update when your question is answered via DirectTrac, and you can login and view your incident history 24x7. Simply visit
https://www.syncfusion.com/account/login?ReturnUrl=%2fsupport%2fdirecttrac%2fincidentslogin. If you have any further questions, we hope you will take advantage of your existing account(if any) and available services as a evaluator.
Please let us know if you have any queries.
Regards,
Bala.