When placing a beacon, or placing a block that would cause a beacon to activate, it will take a few seconds to update, similar to how changing the beam color used to work. Also occurs when deactivating it.
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The beacon update require to fully check the pyramid. You definitively don't want to do that every ticks. The update currently happen every 80 ticks, and this is fine.

I think this ticket is about initial placement only.

No, it's about any time it should activate. Even if they just fixed the lag when actually placing the beacon though, by adding a separate check when it's placed, that would be helpful.
It's too expensive to scan for the complete shape constantly.

I can understand that for the pyramid, but why does it occur when the beacon is first placed?

It updates once right after placement now.

So it does. Not sure when that change happened. Ok then, that's a lot better.

This was partwise fixed in 16w02a. When you pleace the beacon block the beam appears instantly. The same goes for colored glass. When you however place the beacon block on a unfinished pyramid and then complete it, it still has some delay (probably intended).
How is this intended? The color-changing lag was fixed, why not this?
If it's going to be left as it is then the resolution should be "Won't Fix". There's no way this is by design.