Just one thing. I link a lot of external images and it very slow, I think, because it will load all image to check width and height before check existing thumb img.
If possible can it check (isexist)thumb cache image before load external images by using _attribute name, width and height.
It will solve very slow from very larg external images.
I know about this problem. It will fixed in future versions. Now you can uncomment fragment from line 169 to 183. And create directory "remote" in directory with thumbnails.