I had an issue that caused significant loading times during the 'Loading Sprites' phase in the form of hangups that would last between 3 and 10 minutes. This was revealed to be coupled along side an error in the 'factorio-current.log' file along the lines of;
142.926 Warning! Sprite at {0,0; 32x32} from <mod_name>/graphics/icons/<image_name>.png is defined with 4 mipmap levels, but level 1 is expected to be at {32,0; 16x16} which is out of bounds of the source image.
In an effort to solve this issue I found that adding the line, 'icon_mipmaps = nil' where the icon is declared - adjusted to match the code's variant of declaring it - seemed to fix this issue, at least for me.

Yours is one of the mods I had the error with, and was able to fix using that method so I'm offering that information to you. :)
For this case it was the icon 'icon_64.png' declared in 'data-final-fixes.lua'

Update: It has since come to my attention that the hangups caused by this error may have been related to the update prior to 0.18.38. While these errors do exist and this solution may fix them, the update may have solved the slow loading, making this a less immediate issue.