GIMP Version: 2.8.2
Operating System: Windows
OS Version: win7
GIMP Experience: Intermediate Level

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Is it possible to use and animated gif as the splash? I tried using .gif and .apng but lose my animation
Thanks

I have animated GIFs as gimp splashes, I tried as 'heavy' gif files as 16MB. Anything heavier on my machine just results in GIMP loading without splash at all.
5-6 MB GIFs are usually OK and don't increase GIMP start-up time.
APNG is a no-no, since GTK, which actually triggers the GIMP start-up and loads splash images, does not support APNG or animated MNG file format. Even if you have APNG plugin installed.
GTK is limited to jpeg, png, gif and bmp (if I'm not mistaken) file formats only.

I tried the splash above (gimp-splash.gif) and it's only 808KB but gimp loads without any splash

You need to create 'splashes' folder (with lower case s) in gimp Profile directory. It doesn't have to be in Program directory.
You can have as many splash-images there as you want, GTK will pick them randomly at GIMP start-up as long as they have valid file extension.
An official gimp-splash image can stay in its folder or you can move it outside program folder. It's up to you.

thanks k1te, works now