Hi!
On the Caching Options submenu, you can activate the Crop images for caching option, which will create exact size crops of the images for later use: http://i.imgur.com/s54eiu0.png
This might cause a bit of a slowdown at first, but once most of the images are cropped and cached, it should return to the normal speed.
The upcoming version will have an additional option to choose the thumbnail image if available for the featured images.