I do a couple of tests, and all seems to give this: The penalization from weapon is affected by agi!!
The penalty is probably -% based.
The more base aspd you have the less weapon penalty you get.
If you have very low base aspd you get a very high weapon penalty while having high aspd will allow you to equip weapons with low aspd penalty.
All weapons bonus aspd, weapon aspd penalty, potion aspd and skill aspd is reducing the gap between your base aspd (depending on your agi/dex) and the max point (not sure if they use 193 or 200 as max point).
However the alcabringer still gives +1 aspd per 2 refines so while getting aspd higher than 190~191 is incredibly hard with normal buffs/items/weapons this weapon gives you a fixed aspd bonus so its actually a quite powerful weapon now.
EDIT : Murasame gives +2 flat aspd regardless of the users base aspd.