When holding use on an item, there is a base 'use time' which is stored and then decremented every tick while it's being held. Minecraft will only block an attack after the shield has been raised for more than 5 ticks. It checks this by looking at the difference between the current use time countdown and the base use time of the item.
(from LivingEntity's isBlocking() method)
return item.getUseDuration(this.useItem) - this.useItemRemaining >= 5;
This mod uses a Mixin to change that number '5' to whatever you configure in the mod's config.
I think the reason Mojang did this was for balance reasons. When it comes to singleplayer, though, having the delay set to 0 is really great for the more high-energy gameplay style encouraged by adventure modpacks and allows for some super clutch moments.
Raise Time(From 0 to 5. default: 0) The amount of time, in ticks, from when you right-click before the game will block attacks.
Enable(Default: true) Whether or not the effect of the mod is enabled.
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Setting Raise Time to 0 makes shields capable of blocking immediately.
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Setting Raise Time to 5 is the same as vanilla.
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The client-side raise animation takes roughly one tick to complete. So setting Raise Time to 1 or 2 is a decent balance between responsiveness and not being able to blockhit as easily.