Since Physical Crit. is one of the most important stats to look for as an Assassin, lets look at how your critical rate works.
Every 10 Physical Crit. bonus stats you add to your character is equal to around 1% increase in your critical chance. However that is only before you reach a 400 critical rating; so at 400 critical rate you have prox 40% crit rating.
After your critical rating is above 400, every 10-bonus physical crit. rate you addto your character would only result in a 0.5% increase in your crit. chances. The difference is small but you will notice a difference in how fast you kill the mobs.
Attack Speed Increase:
Attack speed is also a very important stat to focus on as an Assassin. Attack speed not only increases how fast you attack but it also influences your engraving chain skill, like Beast Roar, Beast kick, etc. The higher your attack speed is the faster you can unload your skill, and the faster your engraving skills refresh for you to reuse them. This is very important in 1v1 PvP against any class.
As of right now you can only increase your attack speed with certain weapons that have the bonus increase Atk speed stats, or Flurry I - your level 20 Stigma skill, or Chanter Buffs, and lastly via Atk Speed Scrolls.