This property is set to zero by default, meaning virtualization is always enabled. This improves binding performance and memory requirements, at the expense of a small performance decrease while scrolling. If your grid has a small number of rows (about 50 to 100), you may be able to improve scrolling performance by setting this property to a slightly higher value (like 150). This will disable virtualization and will slow down binding, but may improve perceived scroll performance. Setting this property to values higher than 200 is not recommended. Loading times will become too long; the grid will freeze for a few seconds while creating cells for all rows, and the browser will become slow because of the large number of elements on the page.

Reference

FlexSheet Class
FlexSheet Members