Right now every single time you show a texture it's read from your drive.
I tried to optimise my textures as much as I could, instead of grouping all related textures into atlases, I split them into individual files to make files as small as possible.
For example, I used to have a massive fps drop, when many buttons were shown on the screen at once. Button's border, highlight and pushed textures used to be one atlas, 16KB in size. My button border is a set of 8 textures, so when the game had to render 60+ buttons * 8 textures * 16KB each at once, it had to read ~7.5MB from my drive,
that's way too much. I split my atlas into individual textures, but I went even further and also split button border into 8 individual textures, each has a size of ~280
bytes!!!, so now, when I open a spell book, and all buttons are shown, I get no fps drop at all.
You use really big and heavy textures, each is 1MB, ofc when game loads them, you'll get a freeze.
-- update #1
I tweaked your TGAs a bit, cuz 1MB is just wrong for such simple textures. I managed to slim them down to this state.
But they're still 512x512 textures, you'll be able to improve their sizes even more by resizing them to 256 x 256, I dun think that 512 x 512 is really that needed.
-- update #2
256s.
-- update #3
I downloaded your addon, reverted SetAlpha to Show/Hide and used my 256 x 256 TGAs, I dun get any fps drop at all.
Here's an archive w/ textures.
https://www.dropbox.com/s/gny8knthicga5y4/TGA.zip?dl=0