piXflow internaly saves thumbnail versions of your images. If you start piXflow the next time these will be already available an can be showed much faster. In the Dialog "File", "Global Settings ..." you can choose the directory and the size that should be used to save. Recomended values for the size are 200 or 300 pixels depending on how big you want to preview your thumbs.
If you are working with big image file (i.e. from your digital camera) the thumb building can be slow for the first time, so just be patient.