Hi Lori,
Thanks for bringing this issue to our attention. I have notified the development team in this regard, and have logged defect 613 on this issue. We will try our very best to address the same at the earliest. You could track the progress of this bug fix at this link below :
Defect #613 - Horizontal scrollbar scrolls very slowly even when RepeatClickDelay value is set very low
We appreciate your feedback and thanks for your patience.
Regards,
Guru Patwal
Syncfusion, Inc.