The .5 seconds is too much of a delay, but like shields, manually blocking with melee weapons should haev a speed rating. Only the lightest weapons should be able to block instantaneously (like 100 shield speed or above). There should be a speed difference in getting your block up if you have a great maul, versus a longsword.
This.
As oprah approved (I like when some people don't get the joke), attack speed already matters in "block speed" (mindfvck). It's not always enough, but I would be extremely careful with that setting, as it can ruin combat entirely.