FlexCelPreview holds a collection of generated images for the pages it displays, so it can render them faster. By changing MaxBmpCache you can decide how many images you want to hold in memory at the same time. The default is 10. 

This property has no effect when ZoomPreview is larger than 100% On this case, the Bmp cache will have only the current page to preserve resources. 

TFlexCelPreview Properties