A leader ready button.

This would give advantage to active clans over less active ones.

You say that you fill the CCs of your clan castle in 40 minutes. But other clans need more tike.

Other clans like to take their time to study to enemy bases.
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Let us say, 2 active clans got matched up against each other and they were ready after 3 hours of preparation. Well, good for them.

Now, let's imagine that an active clan got matched against a less active clan or simply another active clan which doesn't want to start the war soon.

This way, Active clans will have advantage over other active clans because of luck.

I agree with this. The reason being like the OP said, instead of going the whole preparation day when both clans feel confident. If one clan doesn't say they are ready, then the 48 hours of a normal war goes the same.

I don't think sc will let you start wars and finish them any quicker, clans need to take their time to study and plan their attacks before battle starts. of course donating their cc takes time. all you need is patients:) and be ready when it starts. anyways I think that idea is ruled out.