Say you have a weapon with a 4 second delay and you have zero haste. What will happen is your auto attack will try to fire every 4 seconds. If you are in the middle of doing a combat art then it will queue until your combat art finishes. If you are continually chaining combat arts then you are not giving your auto attack a chance to fire and hence losing dps.
What you need to do is type /weapon in your chat window and it will tell you the actual delay of your weapon in your current group taking into account haste modifiers. What you do then is work out how many combat arts you squeeze in between these timings. if you do any combination of CAs that takes longer than 2 seconds to complete then you delay your auto attack which is bad.
It gets more complicated when you duel wield weapons wit different delays. They become out of synch and it becomes harder to time CAs in between. It is easier if you have two weapons with the same delay
Then you have to factor in temporary haste buffs like Chime of Blades or when items you are wearing proc haste. You need to know when this is happens and what your delay is during that time so that you can modify your timings accordingly.
There are lots of threads on this already - Read
Guard DPS -
Pay particular attention to any posts by Jaraxxx, Saracen or Skel
Good luck
