What is up with how some spawn mini-speedup takes into effect immediately and others slowly?

What is up with how some spawn mini-speedup takes into effect immediately and others slowly?

When one has their base being wrecked and I've/Teammates have taken hits to our HP, we go to base and wait for the speedup. This sometimes takes a second or two or sometimes like 10 seconds which feels like an eternity when you really really need to go back into the team fight to intercept the enemy team.

This has always been troubling me.