Follow-up: this only seems to be the case when the "Animated Great Red Spot" image is in view.

May I ask what GPU do you have?

512MB ATI AMD Radeon Graphics

I'm afraid the only thing I can do in such case is to display a static image instead of a shader. Would you prefer that?

I was able to view it smoothly on my phone, so I'm not too fussed, but that might be a better experience for anyone else that has the same issue in future.

It opened instantly and worked smoothly in Firefox on my 8 year old android.

Yes, on my phone it's fine but on my laptop it's a nightmare.