There is no reason for any other shooter to run down the stairs in that same time frame.
How about not being caught and arrested. Subsequently prosecuted and convicted of the murder of John F. Kennedy. Finally being executed in the electric-chair in Huntsville Texas!
Just when I think your posts cannot get any more ridiculous, you come up with that.
As per usual, you show us once again than any person can only consider to be "logical" what his own mind can process. When you are incapable to understand and/or accept alternative scenarios, you will keep going round in the same circle all the time and always end up at the same flawed conclusion, which is exactly what you are doing here yet again.
It has already been explained to you. If Oswald was the killer, he would have needed to come down the stairs within a 90 seconds because otherwise Baker and Truly could not have seen him in the 2nd floor lunchroom.
Any other killer, who was not seen by Baker and Truly, or anybody else for that matter, would not have the requirement to come down the stairs so quickly. He could have hidden somewhere in the building and, posing as an law enforcement officer, use the commotion to leave the building later. If you don't get a simple concept as this, there is not much anybody can do for you. Ignorance is bliss, right?