A temporary workaround is to use the share button to share the post to your web browser, which opens it there and (after giving it a moment to load them all) plays them at full speed.

I use Thunder as well and noticed this issue. Anywhere in the app where there are multiple GIFs loaded at once (a post like this, the Manage Media screen, etc.), the GIFs render at a more slow speed the more there are:

• This post we are commenting on has 48 GIFs, and is nearly unusable
• This post has 24, and is closer to full speed, but still noticeably slow: https://feddit.org/post/6785713
• This post has 12, and they don’t seem slow to me at all: https://feddit.org/post/6368045

This is the same for every post I have created with 12, 24, or 48 GIFs.