It's because you are downloading the images directly into the List. Downloading them all as a one-off operation on the first-ever run of the App will help, but pre-defining a completed list would be much faster, assuming the List is not going to be edited by the App User.
I see your List is Static, 195 country flags and their related ISO country code? If you can send me a text file (.txt) of the codes and the flag images numbered to synch with the country code I have a desktop program that can convert them into an HTML table (two columns).
You can't upload a zip file of the flags, but you can change the ext to .aia - that should work
You could also use @Kevinkun 's Tableviewer extension:
Not tested, but you should be able to set the img src to a datauri to use base64 strings from a text file, instead of images. (data:image/png;base64,/Vsdofihwoefjsd.....)