Sometimes, creators are either lazy or forget to create proper thumbnails, so the game has to load a couple of big images, just to show them as small previews. Without changing the mod or removing it, you can't do anything against it.
This is VERY interesting, Herb! Thank you for turning my nose onto this! Here's what I have learned so far...
Take 73lac7c's head mods, every time I scroll to them, my game slows a bit. Upon unpacking all 20 of the head mods I have for this author, I found many of them to have lots of JUNK files zipped up within!
I deleted all but the abdata and manifest.xml, according to how a zipmod is supposed to be structured. THEN, I went looking to see just how big the thumbnails 73lac7c used were.
Just as you said, Herby, the thumbnail unity3d files were way bigger than they need to be. I went through all 20 of 73lac7c's mods and "cleaned" them, as well as use an app called SB3Utility(found in a Honey Select 2 Better Repack Tools folder) to open then compress the shit out of their thumbnail containers.
This Mia Winters head mod for example, I compressed the entire textures file from 78MB to under 22MB.
Bottom line, I stopped the brief framerate slowdown when scrolling on my custom heads window. It's kind of a pain to recreate a person's mods fsfs. But if the mods are important to you, this workflow will get them working much better. Now for Milk217's!