|I should have asked....since I know I'm not always right (don't I wish.....lol)....are you using roaming profiles?|
In the original post you said:
"But some time user login take few seconds
But some time its very slow and time taking"
If you use roaming profiles, the times it's faster could be when you're loading a locally cached profile due to not being able to contact the DC. You'll know if that's happening as you'll get error messages stating your inability to contact the DC.
Bandwidth usage could also cause slower/faster logons. ie: when it's busy and a lot of bandwidth is being used logon's will take longer....when it's not busy, less time.
DNS may very well be causing you problems.....but if you're having no resolution problems...either internally or externally, I doubt it is DNS. Unless it's consistently taking more than 10 minutes to logon, I really don't think you have a problem.