I'm not speaking to your MOD specifically, just mods that do this in general. This is a neat feature to have but unfortunately it ads some high overhead as all the thumbs on the page are usually served up by php, requiring more php processes and on large forums that's an issue.
If you could figure out how to serve the image thumbs through direct links, that would be much better.
Take a look at this, he has the right idea here, though this will only work with nginx
https://vborg.vbsupport.ru/showthread.php?t=207566