The quality options page allows you to change the quality of the resampling used when showing the thumbnails. Use the slider to adjust between lower (faster) and higher (slower) values. 

The higher the resampling quality, the slower the thumbs will be redrawn. 

If you have Visual Forms activated for a number of large projects, generally the second lowest quality value is acceptable. Modify the until you are happy with the resulting speed and quality for your projects.