When you block it takes a good second or two before you actually start blocking with your shield making it impossible to make split-second blocks with you needing to hold the shield up for 1 or 2 seconds before you want to block.
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I noticed this as well in my worlds. Just thought it was my mind fooling with me but turns out after some testing this is an issue. I haven't tried it on any other version but I've only started having shield issues after updating to 1.14. I don't know if this is intentional to try to nerf shields or if it's just a bug.