As monster said in another thread, to be absolutely certain that a witness cannot double spend against you you must wait for 2 blocks which means that 3 seconds becomes 6 and 1 becomes 2.
There is a fixed cost per block that the network must pay that is independent of transaction volume. At 1 second blocks it becomes 8 MB per day of empty blocks. Having 2 second blocks would save 1.5 GB per year.
That overhead is insignificant for full nodes and doesn't impact 99% of users of light nodes.
One dumb question :
So regarding decentralized exchange , does this mean a deal can be reversed or a UIA can be sold to multiple people at the same time by a witness ?
It means that if you do something irreversible off-chain in response to something you see happen in the head block that the witness could "double sign" and create another block that includes different transactions.
Any witness that would double-sign would get voted out immediately (and their peer witnesses should be configured to ignore that witnesses blocks).
In practice it is a purely theoretical attack and will not happen because the witnesses will be public (non-anonymous) and they would not make much money.
To pull of the attack also requires that the attacker identify YOUR node and intentionally send you a block. If you are not directly connected to the attacker and the attacker does not know where your computer is on the network then the attack becomes technically impractical.
In other words, only the truly paranoid would worry about it. You are far more likely to get struck by lightning.